Cyberterrorism

From Wikipedia, the free encyclopedia

 

Cyberterrorism is the use of the Internet to conduct violent acts that result in, or threaten, loss of life or significant bodily harm, in order to achieve political or ideological gains through threat or intimidation. It is also sometimes considered an act of Internet terrorism where terrorist activities, including acts of deliberate, large-scale disruption of computer networks, especially of personal computers attached to the Internet by means of tools such as computer viruses, computer worms, phishing, and other malicious software and hardware methods and programming scripts.

Cyberterrorism is a controversial term. Some authors opt for a very narrow definition, relating to deployment by known terrorist organizations of disruption attacks against information systems for the primary purpose of creating alarm, panic, or physical disruption. Other authors prefer a broader definition, which includes cybercrime. Participating in a cyberattack affects the terror threat perception, even if it isn't done with a violent approach. By some definitions, it might be difficult to distinguish which instances of online activities are cyberterrorism or cybercrime.

Cyberterrorism can be also defined as the intentional use of computers, networks, and public internet to cause destruction and harm for personal objectives. Experienced cyberterrorists, who are very skilled in terms of hacking can cause massive damage to government systems, hospital records, and national security programs, which might leave a country, community or organization in turmoil and in fear of further attacks. The objectives of such terrorists may be political or ideological since this can be considered a form of terror.

There is much concern from government and media sources about potential damage that could be caused by cyberterrorism, and this has prompted efforts by government agencies such as the Federal Bureau of Investigations (FBI) and the Central Intelligence Agency (CIA) to put an end to cyber attacks and cyberterrorism.

There have been several major and minor instances of cyberterrorism. Al-Qaeda utilized the internet to communicate with supporters and even to recruit new members.^[5] Estonia, a Baltic country which is constantly evolving in terms of technology, became a battleground for cyberterror in April, 2007 after disputes regarding the removal of a WWII soviet statue located in Estonia's capital Tallinn.

Overview

There is debate over the basic definition of the scope of cyberterrorism. These definitions can be narrow such as the use of Internet to attack other systems in the Internet that result to violence against persons or property. They can also be broad, those that include any form of Internetusage by terrorists ro conventional attacks on information technology infrastructures. There is variation in qualification by motivation, targets, methods, and centrality of computer use in the act. U.S. government agencies also use varying definitions and that none of these have so far attempted to introduce a standard that is binding outside of their sphere of influence.

Depending on context, cyberterrorism may overlap considerably with cybercrime, cyberwar or ordinary terrorism. Eugene Kaspersky, founder of Kaspersky Lab, now feels that "cyberterrorism" is a more accurate term than "cyberwar". He states that "with today's attacks, you are clueless about who did it or when they will strike again. It's not cyber-war, but cyberterrorism." He also equates large-scale cyber weapons, such as the Flame Virus and NetTraveler Virus which his company discovered, to biological weapons, claiming that in an interconnected world, they have the potential to be equally destructive.

If cyberterrorism is treated similarly to traditional terrorism, then it only includes attacks that threaten property or lives, and can be defined as the leveraging of a target's computers and information, particularly via the Internet, to cause physical, real-world harm or severe disruption of infrastructure.

Many academics and researchers who specialize in terrorism studies suggest that cyberterrorism does not exist and is really a matter of hacking or information warfare. They disagree with labeling it as terrorism because of the unlikelihood of the creation of fear, significant physical harm, or death in a population using electronic means, considering current attack and protective technologies.

If death or physical damage that could cause human harm is considered a necessary part of the cyberterrorism definition, then there have been few identifiable incidents of cyberterrorism, although there has been much policy research and public concern. Modern terrorism and political violence is not easily defined, however, and some scholars assert that it is now "unbounded" and not exclusively concerned with physical damage 

There is an old saying that death or loss of property are the side products of terrorism, the main purpose of such incidents is to create terror in peoples' minds and harm bystanders. If any incident in cyberspace can create terror, it may be rightly called cyberterrorism. For those affected by such acts, the fears of cyberterrorism are quite real.

As with cybercrime in general, the threshold of required knowledge and skills to perpetrate acts of cyberterror has been steadily diminishing thanks to freely available hacking suites and online courses. Additionally, the physical and virtual worlds are merging at an accelerated rate, making for many more targets of opportunity which is evidenced by such notable cyber attacks as Stuxnet, the Saudi petrochemical sabotage attempt in 2018 and others.

Defining cyberterrorism

Assigning a concrete definition to cyberterrorism can be hard, due to the difficulty of defining the term terrorism itself. Multiple organizations have created their own definitions, most of which are overly broad. There is also controversy concerning overuse of the term, hyperbole in the media and by security vendors trying to sell "solutions".

One way of understanding cyberterrorism involves the idea that terrorists could cause massive loss of life, worldwide economic chaos and environmental damage by hacking into critical infrastructure systems. The nature of cyberterrorism covers conduct involving computer or Internet technology that:

1. is motivated by a political, religious or ideological cause
2. is intended to intimidate a government or a section of the public to varying degrees
3. seriously interferes with infrastructure

The term "cyberterrorism" can be used in a variety of different ways, but there are limits to its use. An attack on an Internet business can be labeled cyberterrorism, however when it is done for economic motivations rather than ideological it is typically regarded as cybercrime. Convention also limits the label "cyberterrorism" to actions by individuals, independent groups, or organizations. Any form of cyberwarfare conducted by governments and states would be regulated and punishable under international law.

The Technolytics Institute defines cyberterrorism as

"[t]he premeditated use of disruptive activities, or the threat thereof, against computers and/or networks, with the intention to cause harm or further social, ideological, religious, political or similar objectives. Or to intimidate any person in furtherance of such objectives."

The term appears first in defense literature, surfacing (as "cyber-terrorism") in reports by the U.S. Army War College as early as 1998.

The National Conference of State Legislatures, an organization of legislators created to help policymakers in the United States of America with issues such as economy and homeland security defines cyberterrorism as:

[T]he use of information technology by terrorist groups and individuals to further their agenda. This can include use of information technology to organize and execute attacks against networks, computer systems and telecommunications infrastructures, or for exchanging information or making threats electronically. Examples are hacking into computer systems, introducing viruses to vulnerable networks, web site defacing, Denial-of-service attacks, or terroristic threats made via electronic communication.

NATO defines cyberterrorism as "[a] cyberattack using or exploiting computer or communication networks to cause sufficient destruction or disruption to generate fear or to intimidate a society into an ideological goal".

The United States National Infrastructure Protection Center defined cyberterrorism as:

A criminal act perpetrated by the use of computers and telecommunications capabilities resulting in violence, destruction, and/or disruption of services to create fear by causing confusion and uncertainty within a given population, with the goal of influencing a government or population to conform to a political, social, or ideological agenda.

The FBI, another United States agency, defines "cyber terrorism" as “premeditated, politically motivated attack against information, computer systems, computer programs, and data which results in violence against non-combatant targets by subnational groups or clandestine agents”.

These definitions tend to share the view of cyberterrorism as politically and/or ideologically inclined. One area of debate is the difference between cyberterrorism and hacktivism. Hacktivism is ”the marriage of hacking with political activism”. Both actions are politically driven and involve using computers, however cyberterrorism is primarily used to cause harm. It becomes an issue because acts of violence on the computer can be labeled either cyberterrorism or hacktivism.

Types of cyberterror capability

The following three levels of cyberterror capability are defined by Monterey group

• Simple-Unstructured: The capability to conduct basic hacks against individual systems using tools created by someone else. The organization possesses little target analysis, command, and control, or learning capability.
• Advanced-Structured: The capability to conduct more sophisticated attacks against multiple systems or networks and possibly, to modify or create basic hacking tools. The organization possesses an elementary target analysis, command and control, and learning capability.
• Complex-Coordinated: The capability for a coordinated attack capable of causing mass-disruption against integrated, heterogeneous defenses (including cryptography). Ability to create sophisticated hacking tools. Highly capable target analysis, command, and control, and organization learning capability.

Concerns

Cyberterrorism is becoming more and more prominent on social media today. As the Internet becomes more pervasive in all areas of human endeavor, individuals or groups can use the anonymity afforded by cyberspace to threaten citizens, specific groups (i.e. with membership based on ethnicity or belief), communities and entire countries, without the inherent threat of capture, injury, or death to the attacker that being physically present would bring. Many groups such as Anonymous, use tools such as denial-of-service attack to attack and censor groups who oppose them, creating many concerns for freedom and respect for differences of thought. 

Many believe that cyberterrorism is an extreme threat to countries' economies, and fear an attack could potentially lead to another Great Depression. Several leaders agree that cyberterrorism has the highest percentage of threat over other possible attacks on U.S. territory. Although natural disasters are considered a top threat and have proven to be devastating to people and land, there is ultimately little that can be done to prevent such events from happening. Thus, the expectation is to focus more on preventative measures that will make Internet attacks impossible for execution.

As the Internet continues to expand, and computer systems continue to be assigned increased responsibility while becoming more complex and interdependent, sabotage or terrorism via the Internet may become a more serious threat and is possibly one of the top 10 events to "end the human race." People have much easier access to illegal involvement within the cyberspace by the ability to access a part of the internet known as the Dark Web. The Internet of Things promises to further merge the virtual and physical worlds, which some experts see as a powerful incentive for states to use terrorist proxies in furtherance of objectives.

Dependence on the internet is rapidly increasing on a worldwide scale, creating a platform for international cyber terror plots to be formulated and executed as a direct threat to national security. For terrorists, cyber-based attacks have distinct advantages over physical attacks. They can be conducted remotely, anonymously, and relatively cheaply, and they do not require significant investment in weapons, explosive and personnel. The effects can be widespread and profound. Incidents of cyberterrorism are likely to increase. They will be conducted through denial of service attacks, malware, and other methods that are difficult to envision today. One example involves the deaths involving the Islamic State and the online social networks Twitter, Google, and Facebook lead to legal action being taken against them, that ultimately resulted in them being sued.

In an article about cyber attacks by Iran and North Korea, The New York Times observes, "The appeal of digital weapons is similar to that of nuclear capability: it is a way for an outgunned, outfinanced nation to even the playing field. 'These countries are pursuing cyberweapons the same way they are pursuing nuclear weapons,' said James A. Lewis, a computer security expert at the Center for Strategic and International Studies in Washington. 'It's primitive; it's not top of the line, but it's good enough and they are committed to getting it.'"

History

Public interest in cyberterrorism began in the late 1990s, when the term was coined by Barry C. Collin.^[35] As 2000 approached, the fear and uncertainty about the millennium bug heightened, as did the potential for attacks by cyber terrorists. Although the millennium bug was by no means a terrorist attack or plot against the world or the United States, it did act as a catalyst in sparking the fears of a possibly large-scale devastating cyber-attack. Commentators noted that many of the facts of such incidents seemed to change, often with exaggerated media reports.

The high-profile terrorist attacks in the United States on September 11, 2001 and the ensuing War on Terror by the US led to further media coverage of the potential threats of cyberterrorism in the years following. Mainstream media coverage often discusses the possibility of a large attack making use of computer networks to sabotage critical infrastructures with the aim of putting human lives in jeopardy or causing disruption on a national scale either directly or by disruption of the national economy.

Authors such as Winn Schwartau and John Arquilla are reported to have had considerable financial success selling books which described what were purported to be plausible scenarios of mayhem caused by cyberterrorism. Many critics claim that these books were unrealistic in their assessments of whether the attacks described (such as nuclear meltdowns and chemical plant explosions) were possible. A common thread throughout what critics perceive as cyberterror-hype is that of non-falsifiability; that is, when the predicted disasters fail to occur, it only goes to show how lucky we've been so far, rather than impugning the theory. 

In 2016, for the first time ever, the Department of Justice charged Ardit Ferizi with cyberterrorism. He is accused of allegedly hacking into a military website and stealing the names, addresses, and other personal information of government and military personnel and selling it to ISIS.

On the other hand, it is also argued that, despite substantial studies on cyberterrorism, the body of literature is still unable to present a realistic estimate of the actual threat. For instance, in the case of a cyberterrorist attack on a public infrastructure such as a power plant or air traffic control through hacking, there is uncertainty as to its success because data concerning such phenomena are limited.

International attacks and response

Conventions

As of 2016 there have been seventeen conventions and major legal instruments that specifically deal with terrorist activities and can also be applied to cyber terrorism.

• 1963: Convention on Offences and Certain Other Acts Committed on Board Aircraft
• 1970: Convention for the Suppression of Unlawful Seizure of Aircraft
• 1971: Convention for the Suppression of Unlawful Acts Against the Safety of Civil Aviation
• 1973: Convention on the Prevention and Punishment of Crimes against Internationally Protected Persons
• 1979: International Convention against the Taking of Hostages
• 1980: Convention on the Physical Protection of Nuclear Material
• 1988: Protocol for the Suppression of Unlawful Acts of Violence at Airports Serving International Civil Aviation
• 1988: Protocol for the Suppression of Unlawful Acts against the Safety of Fixed Platforms Located on the Continental Shelf
• 1988: Convention for the Suppression of Unlawful Acts against the Safety of Maritime Navigation
• 1989: Supplementary to the Convention for the Suppression of Unlawful Acts against the Safety of Civil Aviation
• 1991: Convention on the Marking of Plastic Explosives for the Purpose of Detection
• 1997: International Convention for the Suppression of Terrorist Bombings
• 1999: International Convention for the Suppression of the Financing of Terrorism
• 2005: Protocol to the Convention for the Suppression of Unlawful Acts against the Safety of Maritime Navigation
• 2005: International Convention for the Suppression of Acts of Nuclear Terrorism
• 2010: Protocol Supplementary to the Convention for the Suppression of Unlawful Seizure of Aircraft
• 2010: Convention on the Suppression of Unlawful Acts Relating to International Civil Aviation

Motivations for cyberattacks

There are many different motives for cyberattacks, with the majority being for financial reasons. However, there is increasing evidence that hackers are becoming more politically motivated. Cyberterrorists are aware that governments are reliant on the internet and have exploited this as a result. For example, Mohammad Bin Ahmad As-Sālim's piece '39 Ways to Serve and Participate in Jihad' discusses how an electronic jihad could disrupt the West through targeted hacks of American websites, and other resources seen as anti-Jihad, modernist, or secular in orientation (Denning, 2010; Leyden, 2007).

International institutions

As of 2016 the United Nations only has one agency that specializes in cyberterrorism, the International Telecommunications Union.

U.S. military/protections against cyberterrorism

The US Department of Defense (DoD) charged the United States Strategic Command with the duty of combating cyberterrorism. This is accomplished through the Joint Task Force-Global Network Operations, which is the operational component supporting USSTRATCOM in defense of the DoD's Global Information Grid. This is done by integrating GNO capabilities into the operations of all DoD computers, networks, and systems used by DoD combatant commands, services and agencies.

On November 2, 2006, the Secretary of the Air Force announced the creation of the Air Force's newest MAJCOM, the Air Force Cyber Command, which would be tasked to monitor and defend American interest in cyberspace. The plan was however replaced by the creation of Twenty-Fourth Air Force which became active in August 2009 and would be a component of the planned United States Cyber Command.

On December 22, 2009, the White House named its head of computer security as Howard Schmidt to coordinate U.S Government, military and intelligence efforts to repel hackers. He left the position in May, 2012. Michael Daniel was appointed to the position of White House Coordinator of Cyber Security the same week and continues in the position during the second term of the Obama administration.

More recently, Obama signed an executive order to enable the US to impose sanctions on either individuals or entities that are suspected to be participating in cyber related acts. These acts were assessed to be possible threats to US national security, financial issues or foreign policy issues. U.S. authorities indicted a man over 92 cyberterrorism hacks attacks on computers used by the Department of Defense. A Nebraska-based consortium apprehended four million hacking attempts in the course of eight weeks. In 2011 cyberterrorism attacks grew 20%.

Estonia and NATO

The Baltic state of Estonia was the target of a massive denial-of-service attack that ultimately rendered the country offline and shut out from services dependent on Internet connectivity in April 2007. The infrastructure of Estonia including everything from online banking and mobile phone networks to government services and access to health care information was disabled for a time. The tech-dependent state experienced severe turmoil and there was a great deal of concern over the nature and intent of the attack. 

The cyber attack was a result of an Estonian-Russian dispute over the removal of a bronze statue depicting a World War II-era Soviet soldier from the center of the capital, Tallinn. In the midst of the armed conflict with Russia, Georgia likewise was subject to sustained and coordinated attacks on its electronic infrastructure in August 2008. In both of these cases, circumstantial evidence point to coordinated Russian attacks, but attribution of the attacks is difficult; though both the countries blame Moscow for contributing to the cyber attacks, proof establishing legal culpability is lacking. 

Estonia joined NATO in 2004, which prompted NATO to carefully monitor its member state's response to the attack. NATO also feared escalation and the possibility of cascading effects beyond Estonia's border to other NATO members. In 2008, directly as a result of the attacks, NATO opened a new center of excellence on cyberdefense to conduct research and training on cyber warfare in Tallinn.

The chaos resulting from the attacks in Estonia illustrated to the world the dependence countries had on information technology. This dependence then makes countries vulnerable to future cyber attacks and terrorism.

Republic of Korea

According to 2016 Deloitte Asia-Pacific Defense Outlook, South Korea's 'Cyber Risk Score' was 884 out of 1,000 and South Korea is found to be the most vulnerable country to cyber attacks in the Asia-Pacific region. Considering South Korea's high speed internet and cutting edge technology, its cyber security infrastructure is relatively weak. The 2013 South Korea cyberattack significantly damaged the Korean economy. In 2017, a ransomware attack harassed private companies and users, who experienced personal information leakage. Additionally, there were North Korea's cyber attacks which risked national security of South Korea.

In response to this, South Korean government's countermeasure is to protect the information security centres the National Intelligence Agency. Currently, 'cyber security' is one of the major goals of NIS Korea. Since 2013, South Korea had established policies related to National cyber security and trying to prevent cyber crises via sophisticated investigation on potential threats. Meanwhile, scholars emphasise on improving the national consciousness towards cyber attacks as South Korea had already entered the so-called 'hyper connected society'.

China

The Chinese Defense Ministry confirmed the existence of an online defense unit in May 2011. Composed of about thirty elite internet specialists, the so-called "Cyber Blue Team", or "Blue Army", is officially claimed to be engaged in cyber-defense operations, though there are fears the unit has been used to penetrate secure online systems of foreign governments.

Pakistan

Pakistani Government has also taken steps to curb the menace of cyberterrorism and extremist propaganda. National Counter Terrorism Authority (Nacta) is working on joint programs with different NGOs and other cyber security organizations in Pakistan to combat this problem. Surf Safe Pakistan is one such example. Now people in Pakistan can report extremist and terrorist related content online on Surf Safe Pakistan portal. The National Counter Terrorism Authority (NACTA) provides the Federal Government's leadership for the Surf Safe Campaign. In March 2008 an al Qaeda forum posted a training website with six training modules to learn cyberterrorism techniques.

Ukraine

A series of powerful cyber attacks began 27 June 2017 that swamped websites of Ukrainian organizations, including banks, ministries, newspapers and electricity firms.

Examples

An operation can be done by anyone anywhere in the world, for it can be performed thousands of miles away from a target. An attack can cause serious damage to a critical infrastructure which may result in casualties.

Some attacks are conducted in furtherance of political and social objectives, as the following examples illustrate:

• In 1996, a computer hacker allegedly associated with the White Supremacist movement temporarily disabled a Massachusetts ISP and damaged part of the ISP's record keeping system. The ISP had attempted to stop the hacker from sending out worldwide racist messages under the ISP's name. The hacker signed off with the threat: "you have yet to see true electronic terrorism. This is a promise."
• In 1998, Spanish protesters bombarded the Institute for Global Communications (IGC) with thousands of bogus e-mail messages. E-mail was tied up and undeliverable to the ISP's users, and support lines were tied up with people who couldn't get their mail. The protestors also spammed IGC staff and member accounts, clogged their Web page with bogus credit card orders, and threatened to employ the same tactics against organizations using IGC services. They demanded that IGC stop hosting the Web site for the Euskal Herria Journal, a New York-based publication supporting Basque independence. Protestors said IGC supported terrorism because a section on the Web pages contained materials on the terrorist group ETA, which claimed responsibility for assassinations of Spanish political and security officials, and attacks on military installations. IGC finally relented and pulled the site because of the "mail bombings."
• In 1998, ethnic Tamil guerrillas attempted to disrupt Sri Lankan embassies by sending large volumes of e-mail. The embassies received 800 e-mails a day over a two-week period. The messages read "We are the Internet Black Tigers and we're doing this to disrupt your communications." Intelligence authorities characterized it as the first known attack by terrorists against a country's computer systems.
• During the Kosovo conflict in 1999, NATO computers were blasted with e-mail bombs and hit with denial-of-service attacks by hacktivists protesting the NATO bombings. In addition, businesses, public organizations and academic institutes received highly politicized virus-laden e-mails from a range of Eastern European countries, according to reports. Web defacements were also common. After the Chinese Embassy was accidentally bombed in Belgrade, Chinese hacktivists posted messages such as "We won't stop attacking until the war stops!" on U.S. government Web sites.
• Since December 1997, the Electronic Disturbance Theater (EDT) has been conducting Web sit-ins against various sites in support of the Mexican Zapatistas. At a designated time, thousands of protestors point their browsers to a target site using software that floods the target with rapid and repeated download requests. EDT's software has also been used by animal rights groups against organizations said to abuse animals. Electrohippies, another group of hacktivists, conducted Web sit-ins against the WTO when they met in Seattle in late 1999. These sit-ins all require mass participation to have much effect, and thus are more suited to use by activists than by terrorists.
• In 2000, a Japanese investigation revealed that the government was using software developed by computer companies affiliated with Aum Shinrikyo, the doomsday sect responsible for the sarin gas attack on the Tokyo subway system in 1995. "The government found 100 types of software programs used by at least 10 Japanese government agencies, including the Defense Ministry, and more than 80 major Japanese companies, including Nippon Telegraph and Telephone." Following the discovery, the Japanese government suspended use of Aum-developed programs out of concern that Aum-related companies may have compromised security by breaching firewalls. gaining access to sensitive systems or information, allowing invasion by outsiders, planting viruses that could be set off later, or planting malicious code that could cripple computer systems and key data system.
• In March 2013, The New York Times reported on a pattern of cyber attacks against U.S. financial institutions believed to be instigated by Iran as well as incidents affecting South Korean financial institutions that originate with the North Korean government.
• In August 2013, media companies including The New York Times, Twitter and the Huffington Post lost control of some of their websites after hackers supporting the Syrian government breached the Australian Internet company that manages many major site addresses. The Syrian Electronic Army, a hacker group that has previously attacked media organisations that it considers hostile to the regime of Syrian president Bashar al-Assad, claimed credit for the Twitter and Huffington Post hacks in a series of Twitter messages. Electronic records showed that NYTimes.com, the only site with an hours-long outage, redirected visitors to a server controlled by the Syrian group before it went dark.
  
  

• The website of Air Botswana, defaced by a group calling themselves the "Pakistan Cyber Army"
• Pakistani Cyber Army is the name taken by a group of hackers who are known for their defacement of websites, particularly Indian, Chinese, and Israeli companies and governmental organizations, claiming to represent Pakistani nationalist and Islamic interests. The group is thought to have been active since at least 2008, and maintains an active presence on social media, especially Facebook. Its members have claimed responsibility for the hijacking of websites belonging to Acer, BSNL, India's CBI, Central Bank, and the State Government of Kerala.
• British hacker Kane Gamble, sentenced to 2 years in youth detention, posed as CIA chief to access highly sensitive information. He also "cyber-terrorized" high-profile U.S. intelligence officials such as then CIA chief John Brennan or Director of National Intelligence James Clapper. The judge said Gamble engaged in "politically motivated cyber terrorism."

Sabotage

Non-political acts of sabotage have caused financial and other damage. In 2000, disgruntled employee Vitek Boden caused the release of 800,000 litres of untreated sewage into waterways in Maroochy Shire, Australia.

More recently, in May 2007 Estonia was subjected to a mass cyber-attack in the wake of the removal of a Russian World War II war memorial from downtown Tallinn. The attack was a distributed denial-of-service attack in which selected sites were bombarded with traffic to force them offline; nearly all Estonian government ministry networks as well as two major Estonian bank networks were knocked offline; in addition, the political party website of Estonia's Prime Minister Andrus Ansip featured a counterfeit letter of apology from Ansip for removing the memorial statue. Despite speculation that the attack had been coordinated by the Russian government, Estonia's defense minister admitted he had no conclusive evidence linking cyber attacks to Russian authorities. Russia called accusations of its involvement "unfounded", and neither NATO nor European Commission experts were able to find any conclusive proof of official Russian government participation. In January 2008 a man from Estonia was convicted for launching the attacks against the Estonian Reform Party website and fined.

During the Russia-Georgia War, on 5 August 2008, three days before Georgia launched its invasion of South Ossetia, the websites for OSInform News Agency and OSRadio were hacked. The OSinform website at osinform.ru kept its header and logo, but its content was replaced by a feed to the Alania TV website content. Alania TV, a Georgian government-supported television station aimed at audiences in South Ossetia, denied any involvement in the hacking of the websites. Dmitry Medoyev, at the time the South Ossetian envoy to Moscow, claimed that Georgia was attempting to cover up information on events which occurred in the lead-up to the war. One such cyber attack caused the Parliament of Georgia and Georgian Ministry of Foreign Affairs websites to be replaced by images comparing Georgian president Mikheil Saakashvili to Adolf Hitler. Other attacks involved denials of service to numerous Georgian and Azerbaijani websites, such as when Russian hackers allegedly disabled the servers of the Azerbaijani Day.Az news agency.

In June 2019, Russia has conceded that it is "possible" its electrical grid is under cyber-attack by the United States. The New York Times reported that American hackers from the United States Cyber Command planted malware potentially capable of disrupting the Russian electrical grid.

Website defacement and denial of service

Even more recently, in October 2007, the website of Ukrainian president Viktor Yushchenko was attacked by hackers. A radical Russian nationalist youth group, the Eurasian Youth Movement, claimed responsibility.

In 1999 hackers attacked NATO computers. The computers flooded them with email and hit them with a denial-of-service attack. The hackers were protesting against the NATO bombings of the Chinese embassy in Belgrade. Businesses, public organizations and academic institutions were bombarded with highly politicized emails containing viruses from other European countries.

In December 2018, Twitter warned of "unusual activity" from China and Saudi Arabia. A bug was detected in November that could have revealed the country code of users' phone numbers. Twitter said the bug could have had ties to "state-sponsored actors".

Transgenerational epigenetic inheritance

From Wikipedia, the free encyclopedia

 

Genetically identical mice with different DNA methylation patterns causing kinks in the tail of one but not the other.

 

Transgenerational epigenetic inheritance is the transmission of information from one generation of an organism to the next (i.e., parent–child transmission) that affects the traits of offspring without alteration of the primary structure of DNA (i.e., the sequence of nucleotides)—in other words, epigenetically. The less precise term "epigenetic inheritance" may be used to describe both cell–cell and organism–organism information transfer. Although these two levels of epigenetic inheritance are equivalent in unicellular organisms, they may have distinct mechanisms and evolutionary distinctions in multicellular organisms. 

For some epigenetically influenced traits, the epigenetic marks can be induced by the environment and some marks are heritable, leading some to view epigenetics as a relaxation of the rejection of the inheritance of acquired characteristics (Lamarckism).

Epigenetic categories

Four general categories of epigenetic modification are known:

1. self-sustaining metabolic loops, in which a mRNA or protein product of a gene stimulates transcription of the gene; e.g. Wor1 gene in Candida albicans
2. structural templating in which structures are replicated using a template or scaffold structure on the parent; e.g. the orientation and architecture of cytoskeletal structures, cilia and flagella, prions, proteins that replicate by changing the structure of normal proteins to match their own
3. chromatin marks, in which methyl or acetyl groups bind to DNA nucleotides or histones thereby altering gene expression patterns; e.g. Lcyc gene in Linaria vulgaris described below
4. RNA silencing, in which small RNA strands interfere (RNAi) with the transcription of DNA or translation of mRNA; known only from a few studies, mostly in Caenorhabditis elegans

Inheritance of epigenetic marks

Epigenetic variation may take one of four general forms. Others may yet be elucidated, but currently self-sustaining feedback loops, spatial templating, chromatin marking, and RNA-mediated pathways modify epigenes at the level of individual cells. Epigenetic variation within multicellular organisms may be endogenous, generated by cell–cell signaling (e.g. during cell differentiation early in development), or exogenous, a cellular response to environmental cues.

Removal vs. retention

In sexually reproducing organisms, much of the epigenetic modification within cells is reset during meiosis (e.g. marks at the FLC locus controlling plant vernalization), though some epigenetic responses have been shown to be conserved (e.g. transposon methylation in plants). Differential inheritance of epigenetic marks due to underlying maternal or paternal biases in removal or retention mechanisms may lead to the assignment of epigenetic causation to some parent of origin effects in animals and plants.

Reprogramming

In mammals, epigenetic marks are erased during two phases of the life cycle. Firstly just after fertilization and secondly, in the developing primordial germ cells, the precursors to future gametes. During fertilization the male and female gametes join in different cell cycle states and with different configuration of the genome. The epigenetic marks of the male are rapidly diluted. First, the protamines associated with male DNA are replaced with histones from the female's cytoplasm, most of which are acetylated due to either higher abundance of acetylated histones in the female's cytoplasm or through preferential binding of the male DNA to acetylated histones. Second, male DNA is systematically demethylated in many organisms, possibly through 5-hydroxymethylcytosine. However, some epigenetic marks, particularly maternal DNA methylation, can escape this reprogramming; leading to parental imprinting. 

In the primordial germ cells (PGC) there is a more extensive erasure of epigenetic information. However, some rare sites can also evade erasure of DNA methylation. If epigenetic marks evade erasure during both zygotic and PGC reprogramming events, this could enable transgenerational epigenetic inheritance. 

Recognition of the importance of epigenetic programming to the establishment and fixation of cell line identity during early embryogenesis has recently stimulated interest in artificial removal of epigenetic programming. Epigenetic manipulations may allow for restoration of totipotency in stem cells or cells more generally, thus generalizing regenerative medicine.

Retention

Cellular mechanisms may allow for co-transmission of some epigenetic marks. During replication, DNA polymerases working on the leading and lagging strands are coupled by the DNA processivity factor proliferating cell nuclear antigen (PCNA), which has also been implicated in patterning and strand crosstalk that allows for copy fidelity of epigenetic marks. Work on histone modification copy fidelity has remained in the model phase, but early efforts suggest that modifications of new histones are patterned on those of the old histones and that new and old histones randomly assort between the two daughter DNA strands. With respect to transfer to the next generation, many marks are removed as described above. Emerging studies are finding patterns of epigenetic conservation across generations. For instance, centromeric satellites resist demethylation. The mechanism responsible for this conservation is not known, though some evidence suggests that methylation of histones may contribute. Dysregulation of the promoter methylation timing associated with gene expression dysregulation in the embryo was also identified.

Decay

Whereas the mutation rate in a given 100-base gene may be 10⁻⁷ per generation, epigenes may "mutate" several times per generation or may be fixed for many generations. This raises the question: do changes in epigene frequencies constitute evolution? Rapidly decaying epigenetic effects on phenotypes (i.e. lasting less than three generations) may explain some of the residual variation in phenotypes after genotype and environment are accounted for. However, distinguishing these short-term effects from the effects of the maternal environment on early ontogeny remains a challenge.

Contribution to phenotypes

The relative importance of genetic and epigenetic inheritance is subject to debate. Though hundreds of examples of epigenetic modification of phenotypes have been published, few studies have been conducted outside of the laboratory setting. Therefore, the interactions of genes and epigenes with the environment cannot be inferred despite the central role of environment in natural selection. Experimental methodologies for manipulating epigenetic mechanisms are nascent (e.g.) and will need rigorous demonstration before studies explicitly testing the relative contributions of genotype, environment, and epigenotype are feasible.

In plants

b1 paramutation in maize. The B' allele converts the B-I allele to a B'-like state after interaction in F1 heterozygotes. These converted alleles gain the ability to convert naive B-I alleles in subsequent generations resulting in all progeny displaying lightly pigmented phenotype.

 

Studies concerning transgenerational epigenetic inheritance in plants have been reported as early as the 1950s. One of the earliest and best characterized examples of this is b1 paramutation in maize. The b1 gene encodes a basic helix-loop-helix transcription factor that is involved in the anthocyanin production pathway. When the b1 gene is expressed, the plant accumulates anthocyanin within its tissues, leading to a purple coloration of those tissues. The B-I allele (for B-Intense) has high expression of b1 resulting in the dark pigmentation of the sheath and husk tissues while the B' (pronounced B-prime) allele has low expression of b1 resulting in low pigmentation in those tissues. When homozygous B-I parents are crossed to homozygous B', the resultant F1 offspring all display low pigmentation which is due gene silencing of b1. Unexpectedly, when F1 plants are self-crossed, the resultant F2 generation all display low pigmentation and have low levels of b1 expression. Furthermore, when any F2 plant (including those that are genetically homozygous for B-I) are crossed to homozygous B-I, the offspring will all display low pigmentation and expression of b1. The lack of darkly pigmented individuals in the F2 progeny is an example of non-Mendelian inheritance and further research has suggested that the B-I allele is converted to B' via epigenetic mechanisms. The B' and B-I alleles are considered to be epialleles because they are identical at the DNA sequence level but differ in the level of DNA methylation, siRNA production, and chromosomal interactions within the nucleus. Additionally, plants defective in components of the RNA-directed DNA-methylation pathway show an increased expression of b1 in B' individuals similar to that of B-I, however, once these components are restored, the plant reverts to the low expression state. Although spontaneous conversion from B-I to B' has been observed, a reversion from B' to B-I (green to purple) has never been observed over 50 years and thousands of plants in both greenhouse and field experiments.

Examples of environmentally induced transgenerational epigenetic inheritance in plants has also been reported. In one case, rice plants that were exposed to drought-simulation treatments displayed increased tolerance to drought after 11 generations of exposure and propagation by single-seed descent as compared to non-drought treated plants. Differences in drought tolerance was linked to directional changes in DNA-methylation levels throughout the genome, suggesting that stress-induced heritable changes in DNA-methylation patterns may be important in adaptation to recurring stresses. In another study, plants that were exposed to moderate caterpillar herbivory over multiple generations displayed increased resistance to herbivory in subsequent generations (as measured by caterpillar dry mass) compared to plants lacking herbivore pressure. This increase in herbivore resistance persisted after a generation of growth without any herbivore exposure suggesting that the response was transmitted across generations. The report concluded that components of the RNA-directed DNA-methylation pathway are involved in the increased resistance across generations.

In humans

A number of studies suggest the existence of transgenerational epigenetic inheritance in humans. These include those of the Dutch famine of 1944–45, wherein the offspring born during the famine were smaller than those born the year before the famine and the effects could last for two generations. Moreover, these offspring were found to have an increased risk of glucose intolerance in adulthood. Differential DNA methylation has been found in adult female offspring who had been exposed to famine in utero, but it is unknown whether these differences are present in their germline. It is hypothesized that inhibiting the PIM3 gene may have caused slower metabolism in later generations, but causation has not been proven, only correlation. The phenomenon is sometimes referred to as Dutch Hunger Winter Syndrome. Another study hypothesizes epigenetic changes on the Y chromosome to explain differences in lifespan among the male descendants of prisoners of war in the American Civil War.

The Överkalix study noted sex-specific effects; a greater body mass index (BMI) at 9 years in sons, but not daughters, of fathers who began smoking early. The paternal grandfather's food supply was only linked to the mortality RR of grandsons and not granddaughters. The paternal grandmother's food supply was only associated with the granddaughters' mortality risk ratio. When the grandmother had a good food supply was associated with a twofold higher mortality (RR). This transgenerational inheritance was observed with exposure during the slow growth period (SGP). The SGP is the time before the start of puberty, when environmental factors have a larger impact on the body. The ancestors' SGP in this study was set between the ages of 9-12 for boys and 8–10 years for girls. This occurred in the SGP of both grandparents, or during the gestation period/infant life of the grandmothers, but not during either grandparent's puberty. The father's poor food supply and the mother's good food supply were associated with a lower risk of cardiovascular death.

The loss of genetic expression which results in Prader–Willi syndrome or Angelman syndrome has in some cases been found to be caused by epigenetic changes (or "epimutations") on both the alleles, rather than involving any genetic mutation. In all 19 informative cases, the epimutations that, together with physiological imprinting and therefore silencing of the other allele, were causing these syndromes were localized on a chromosome with a specific parental and grandparental origin. Specifically, the paternally derived chromosome carried an abnormal maternal mark at the SNURF-SNRPN, and this abnormal mark was inherited from the paternal grandmother.

Similarly, epimutations on the MLH1 gene has been found in two individuals with a phenotype of hereditary nonpolyposis colorectal cancer, and without any frank MLH1 mutation which otherwise causes the disease. The same epimutations were also found on the spermatozoa of one of the individuals, indicating the potential to be transmitted to offspring.

A study has shown childhood abuse (defined in this study as "sexual contact, severe physical abuse and/or severe neglect") leads to epigenetic modifications of glucocorticoid receptor expression which play a role in HPA (hypothalamic-pituitary-adrenal) activity. Animal experiments have shown that epigenetic changes depend on mother-infant interactions after birth. In a recent study investigating correlations among maternal stress in pregnancy and methylation in teenagers and their mothers, it has been found that children of women who were abused during pregnancy were significantly more likely than others to have methylated glucocorticoid-receptor genes, which in turn change the response to stress, leading to a higher susceptibility to anxiety.

Effects on fitness

Epigenetic inheritance may only affect fitness if it predictably alters a trait under selection. Evidence has been forwarded that environmental stimuli are important agents in the alteration of epigenes. Ironically, Darwinian evolution may act on these neo-Lamarckian acquired characteristics as well as the cellular mechanisms producing them (e.g. methyltransferase genes). Epigenetic inheritance may confer a fitness benefit to organisms that deal with environmental changes at intermediate timescales. Short-cycling changes are likely to have DNA-encoded regulatory processes, as the probability of the offspring needing to respond to changes multiple times during their lifespans is high. On the other end, natural selection will act on populations experiencing changes on longer-cycling environmental changes. In these cases, if epigenetic priming of the next generation is deleterious to fitness over most of the interval (e.g. misinformation about the environment), these genotypes and epigenotypes will be lost. For intermediate time cycles, the probability of the offspring encountering a similar environment is sufficiently high without substantial selective pressure on individuals lacking a genetic architecture capable of responding to the environment. Naturally, the absolute lengths of short, intermediate, and long environmental cycles will depend on the trait, the length of epigenetic memory, and the generation time of the organism. Much of the interpretation of epigenetic fitness effects centers on the hypothesis that epigenes are important contributors to phenotypes, which remains to be resolved.

Deleterious effects

Inherited epigenetic marks may be important for regulating important components of fitness. In plants, for instance, the Lcyc gene in Linaria vulgaris controls the symmetry of the flower. Linnaeus first described radially symmetric mutants, which arise when Lcyc is heavily methylated. Given the importance of floral shape to pollinators, methylation of Lcyc homologues (e.g. CYCLOIDEA) may have deleterious effects on plant fitness. In animals, numerous studies have shown that inherited epigenetic marks can increase susceptibility to disease. Transgenerational epigenetic influences are also suggested to contribute to disease, especially cancer, in humans. Tumor methylation patterns in gene promotors have been shown to correlate positively with familial history of cancer. Furthermore, methylation of the MSH2 gene is correlated with early-onset colorectal and endometrial cancers.

Putatively adaptive effects

Experimentally demethylated seeds of the model organism Arabidopsis thaliana have significantly higher mortality, stunted growth, delayed flowering, and lower fruit set, indicating that epigenes may increase fitness. Furthermore, environmentally induced epigenetic responses to stress have been shown to be inherited and positively correlated with fitness. In animals, communal nesting changes mouse behavior increasing parental care regimes and social abilities that are hypothesized to increase offspring survival and access to resources (such as food and mates), respectively.

Macroevolutionary patterns

Inherited epigenetic effects on phenotypes have been documented in bacteria, protists, fungi, plants, and animals. Though no systematic study of epigenetic inheritance has been conducted (most focus on model organisms), there is preliminary evidence that this mode of inheritance is more important in plants than in animals. The early differentiation of animal germlines is likely to preclude epigenetic marking occurring later in development, while in plants and fungi somatic cells may be incorporated into the germ line.

Life history patterns may also contribute to the occurrence of epigenetic inheritance. Sessile organisms, those with low dispersal capability, and those with simple behavior may benefit most from conveying information to their offspring via epigenetic pathways. Geographic patterns may also emerge, where highly variable and highly conserved environments might host fewer species with important epigenetic inheritance.

Controversies

Humans have long recognized that traits of the parents are often seen in offspring. This insight led to the practical application of selective breeding of plants and animals, but did not address the central question of inheritance: how are these traits conserved between generations, and what causes variation? Several positions have been held in the history of evolutionary thought.

Blending vs. particulate inheritance

Blending inheritance leads to the averaging out of every characteristic, which as the engineer Fleeming Jenkin pointed out, makes evolution by natural selection impossible.

 

Addressing these related questions, scientists during the time of the Enlightenment largely argued for the blending hypothesis, in which parental traits were homogenized in the offspring much like buckets of different colored paint being mixed together. Critics of Charles Darwin's On the Origin of Species, pointed out that under this scheme of inheritance, variation would quickly be swamped by the majority phenotype. In the paint bucket analogy, this would be seen by mixing two colors together and then mixing the resulting color with only one of the parent colors 20 times; the rare variant color would quickly fade.

Unknown to most of the European scientific community, a monk by the name of Gregor Mendel had resolved the question of how traits are conserved between generations through breeding experiments with pea plants. Charles Darwin thus did not know of Mendel's proposed "particulate inheritance" in which traits were not blended but passed to offspring in discrete units that we now call genes. Darwin came to reject the blending hypothesis even though his ideas and Mendel's were not unified until the 1930s, a period referred to as the modern synthesis.

Inheritance of innate vs. acquired characteristics

In his 1809 book, Philosophie Zoologique, Jean-Baptiste Lamarck recognized that each species experiences a unique set of challenges due to its form and environment. Thus, he proposed that the characters used most often would accumulate a "nervous fluid." Such acquired accumulations would then be transmitted to the individual's offspring. In modern terms, a nervous fluid transmitted to offspring would be a form of epigenetic inheritance.

Lamarckism, as this body of thought became known, was the standard explanation for change in species over time when Charles Darwin and Alfred Russel Wallace co-proposed a theory of evolution by natural selection in 1859. Responding to Darwin and Wallace's theory, a revised neo-Lamarckism attracted a small following of biologists, though the Lamarckian zeal was quenched in large part due to Weismann's famous experiment in which he cut off the tails of mice over several successive generations without having any effect on tail length. Thus the emergent consensus that acquired characteristics could not be inherited became canon.

Revision of evolutionary theory

Non-genetic variation and inheritance, however, proved to be quite common. Concurrent to the modern evolutionary synthesis (unifying Mendelian genetics and natural selection), C. H. Waddington was working to unify developmental biology and genetics. In so doing, he coined the word "epigenetic" to represent the ordered differentiation of embryonic cells into functionally distinct cell types despite having identical primary structure of their DNA. Waddington's epigenetics was sporadically discussed, becoming more of a catch-all for puzzling non-genetic heritable characters rather than advancing the body of inquiry. Consequently, the definition of Waddington's word has itself evolved, broadening beyond the subset of developmentally signaled, inherited cell specialization. 

Some scientists have questioned if epigenetic inheritance compromises the foundation of the modern synthesis. Outlining the central dogma of molecular biology, Francis Crick succinctly stated, "DNA is held in a configuration by histone[s] so that it can act as a passive template for the simultaneous synthesis of RNA and protein[s]. None of the detailed 'information' is in the histone." However, he closes the article stating, "this scheme explains the majority of the present experimental results!" Indeed, the emergence of epigenetic inheritance (in addition to advances in the study of evolutionary-development, phenotypic plasticity, evolvability, and systems biology) has strained the current framework of the modern evolutionary synthesis, and prompted the re-examination of previously dismissed evolutionary mechanisms.

There has been much critical discussion of mainstream evolutionary theory by Edward J Steele, Robyn A Lindley and colleagues, Fred Hoyle and N. Chandra Wickramasinghe, Yongsheng Liu Denis Noble, John Mattick and others that the logical inconsistencies as well as Lamarckian Inheritance effects involving direct DNA modifications, as well as the just described indirect, viz. epigenetic, transmissions, challenge conventional thinking in evolutionary biology and adjacent fields.

Mendelian inheritance

From Wikipedia, the free encyclopedia

Gregor Mendel, the Moravian Augustinian monk who founded the modern science of genetics

Mendelian inheritance is a type of biological inheritance that follows the laws originally proposed by Gregor Mendel in 1865 and 1866 and re-discovered in 1900. These laws were initially controversial. When Mendel's theories were integrated with the Boveri–Sutton chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics. Ronald Fisher combined these ideas with the theory of natural selection in his 1930 book The Genetical Theory of Natural Selection, putting evolution onto a mathematical footing and forming the basis for population genetics within the modern evolutionary synthesis.

History

The principles of Mendelian inheritance were named for and first derived by Gregor Johann Mendel, a nineteenth-century Moravian monk who formulated his ideas after conducting simple hybridisation experiments with pea plants (Pisum sativum) he had planted in the garden of his monastery. Between 1856 and 1863, Mendel cultivated and tested some 5,000 pea plants. From these experiments, he induced two generalizations which later became known as Mendel's Principles of Heredity or Mendelian inheritance. He described these principles in a two-part paper, Versuche über Pflanzen-Hybriden (Experiments on Plant Hybridization), that he read to the Natural History Society of Brno on 8 February and 8 March 1865, and which was published in 1866.

Mendel's conclusions were largely ignored by the vast majority. Although they were not completely unknown to biologists of the time, they were not seen as generally applicable, even by Mendel himself, who thought they only applied to certain categories of species or traits. A major block to understanding their significance was the importance attached by 19th-century biologists to the apparent blending of many inherited traits in the overall appearance of the progeny, now known to be due to multi-gene interactions, in contrast to the organ-specific binary characters studied by Mendel. In 1900, however, his work was "re-discovered" by three European scientists, Hugo de Vries, Carl Correns, and Erich von Tschermak. The exact nature of the "re-discovery" has been debated: De Vries published first on the subject, mentioning Mendel in a footnote, while Correns pointed out Mendel's priority after having read De Vries' paper and realizing that he himself did not have priority. De Vries may not have acknowledged truthfully how much of his knowledge of the laws came from his own work and how much came only after reading Mendel's paper. Later scholars have accused Von Tschermak of not truly understanding the results at all.

Regardless, the "re-discovery" made Mendelism an important but controversial theory. Its most vigorous promoter in Europe was William Bateson, who coined the terms "genetics" and "allele" to describe many of its tenets. The model of heredity was contested by other biologists because it implied that heredity was discontinuous, in opposition to the apparently continuous variation observable for many traits. Many biologists also dismissed the theory because they were not sure it would apply to all species. However, later work by biologists and statisticians such as Ronald Fisher showed that if multiple Mendelian factors were involved in the expression of an individual trait, they could produce the diverse results observed, and thus showed that Mendelian genetics is compatible with natural selection. Thomas Hunt Morgan and his assistants later integrated Mendel's theoretical model with the chromosome theory of inheritance, in which the chromosomes of cells were thought to hold the actual hereditary material, and created what is now known as classical genetics, a highly successful foundation which eventually cemented Mendel's place in history. 

Mendel's findings allowed scientists such as Fisher and J.B.S. Haldane to predict the expression of traits on the basis of mathematical probabilities. An important aspect of Mendel's success can be traced to his decision to start his crosses only with plants he demonstrated were true-breeding. He only measured discrete (binary) characteristics, such as color, shape, and position of the seeds, rather than quantitatively variable characteristics. He expressed his results numerically and subjected them to statistical analysis. His method of data analysis and his large sample size gave credibility to his data. He had the foresight to follow several successive generations (F2, F3) of pea plants and record their variations. Finally, he performed "test crosses" (backcrossing descendants of the initial hybridization to the initial true-breeding lines) to reveal the presence and proportions of recessive characters.

Mendel's laws

A Punnett square for one of Mendel's pea plant experiments – self-fertilization of the F1 generation

 

Mendel discovered that, when he crossed purebred white flower and purple flower pea plants (the parental or P generation), the result was not a blend. Rather than being a mix of the two, the offspring (known as the F₁ generation) was purple-flowered. When Mendel self-fertilized the F₁ generation pea plants, he obtained a purple flower to white flower ratio in the F₂ generation of 3 to 1. The results of this cross are tabulated in the Punnett square to the right.

He then conceived the idea of heredity units, which he called "factors". Mendel found that there are alternative forms of factors—now called genes—that account for variations in inherited characteristics. For example, the gene for flower color in pea plants exists in two forms, one for purple and the other for white. The alternative "forms" are now called alleles. For each biological trait, an organism inherits two alleles, one from each parent. These alleles may be the same or different. An organism that has two identical alleles for a gene is said to be homozygous for that gene (and is called a homozygote). An organism that has two different alleles for a gene is said be heterozygous for that gene (and is called a heterozygote).

Mendel hypothesized that allele pairs separate randomly, or segregate, from each other during the production of gametes: egg and sperm. Because allele pairs separate during gamete production, a sperm or egg carries only one allele for each inherited trait. When sperm and egg unite at fertilization, each contributes its allele, restoring the paired condition in the offspring. This is called the Law of Segregation. Mendel also found that each pair of alleles segregates independently of the other pairs of alleles during gamete formation. This is known as the Law of Independent Assortment.

The genotype of an individual is made up of the many alleles it possesses. An individual's physical appearance, or phenotype, is determined by its alleles as well as by its environment. The presence of an allele does not mean that the trait will be expressed in the individual that possesses it. If the two alleles of an inherited pair differ (the heterozygous condition), then one determines the organism’s appearance and is called the dominant allele; the other has no noticeable effect on the organism’s appearance and is called the recessive allele. Thus, in the example above the dominant purple flower allele will hide the phenotypic effects of the recessive white flower allele. This is known as the Law of Dominance but it is not a transmission law: it concerns the expression of the genotype. The upper case letters are used to represent dominant alleles whereas the lowercase letters are used to represent recessive alleles.

Mendel's laws of inheritance

Law	Definition
Law of segregation	During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.
Law of independent assortment	Genes of different traits can segregate independently during the formation of gametes.
Law of dominance	Some alleles are dominant while others are recessive; an organism with at least one dominant allele will display the effect of the dominant allele.

In the pea plant example above, the capital "B" represents the dominant allele for purple flowers and lowercase "b" represents the recessive allele for white flowers. Both parental plants were true-breeding, and one parental variety had two alleles for purple flowers (BB) while the other had two alleles for white flowers (bb). As a result of fertilization, the F₁ hybrids each inherited one allele for purple flowers and one for white. All the F₁ hybrids (Bb) had purple flowers, because the dominant B allele has its full effect in the heterozygote, while the recessive b allele has no effect on flower color. For the F₂ plants, the ratio of plants with purple flowers to those with white flowers (3:1) is called the phenotypic ratio. The genotypic ratio, as seen in the Punnett square, is 1 BB : 2 Bb : 1 bb.

Law of Segregation of genes (the "First Law")

Figure 1 Dominant and recessive phenotypes.
(1) Parental generation.
(2) F₁ generation.
(3) F₂ generation. Dominant (red) and recessive (white) phenotype look alike in the F₁ (first) generation and show a 3:1 ratio in the F₂ (second) generation.

 

The Law of Segregation states that every individual organism contains two alleles for each trait, and that these alleles segregate (separate) during meiosis such that each gamete contains only one of the alleles. An offspring thus receives a pair of alleles for a trait by inheriting homologous chromosomes from the parent organisms: one allele for each trait from each parent.

Molecular proof of this principle was subsequently found through observation of meiosis by two scientists independently, the German botanist Oscar Hertwig in 1876, and the Belgian zoologist Edouard Van Beneden in 1883. Paternal and maternal chromosomes get separated in meiosis and the alleles with the traits of a character are segregated into two different gametes. Each parent contributes a single gamete, and thus a single, randomly successful allele copy to their offspring and fertilization.

Law of Independent Assortment (the "Second Law")

According to independent assortment, 3 homologous pairs create 8 possible combinations, all equally likely to be fertilized. The equation to determine the number of possible combinations given the number of homologous pairs = 2^x (x = number of homologous pairs)

 

Figure 2 Dihybrid cross. The phenotypes of two independent traits show a 9:3:3:1 ratio in the F₂ generation. In this example, coat color is indicated by B (brown, dominant) or b (white), while tail length is indicated by S (short, dominant) or s (long). When parents are homozygous for each trait (SSbb and ssBB), their children in the F₁ generation are heterozygous at both loci and only show the dominant phenotypes (SsbB). If the children mate with each other, in the F₂ generation all combinations of coat color and tail length occur: 9 are brown/short (purple boxes), 3 are white/short (pink boxes), 3 are brown/long (blue boxes) and 1 is white/long (green box).

 

The Law of Independent Assortment states that alleles for separate traits are passed independently of one another. That is, the biological selection of an allele for one trait has nothing to do with the selection of an allele for any other trait. Mendel found support for this law in his dihybrid cross experiments (Fig. 1). In his monohybrid crosses, an idealized 3:1 ratio between dominant and recessive phenotypes resulted. In dihybrid crosses, however, he found a 9:3:3:1 ratios (Fig. 2). This shows that each of the two alleles is inherited independently from the other, with a 3:1 phenotypic ratio for each. 

Independent assortment occurs in eukaryotic organisms during meiotic metaphase I, and produces a gamete with a mixture of the organism's chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent chromosome along the metaphase plate with respect to the other bivalent chromosomes. Along with crossing over, independent assortment increases genetic diversity by producing novel genetic combinations.

There are many violations of independent assortment due to genetic linkage.

Of the 46 chromosomes in a normal diploid human cell, half are maternally derived (from the mother's egg) and half are paternally derived (from the father's sperm). This occurs as sexual reproduction involves the fusion of two haploid gametes (the egg and sperm) to produce a new organism having the full complement of chromosomes. During gametogenesis—the production of new gametes by an adult—the normal complement of 46 chromosomes needs to be halved to 23 to ensure that the resulting haploid gamete can join with another gamete to produce a diploid organism. An error in the number of chromosomes, such as those caused by a diploid gamete joining with a haploid gamete, is termed aneuploidy. 

In independent assortment, the chromosomes that result are randomly sorted from all possible maternal and paternal chromosomes. Because zygotes end up with a random mix instead of a pre-defined "set" from either parent, chromosomes are therefore considered assorted independently. As such, the zygote can end up with any combination of paternal or maternal chromosomes. Any of the possible variants of a zygote formed from maternal and paternal chromosomes will occur with equal frequency. For human gametes, with 23 pairs of chromosomes, the number of possibilities is 2²³ or 8,388,608 possible combinations. The zygote will normally end up with 23 chromosomes pairs, but the origin of any particular chromosome will be randomly selected from paternal or maternal chromosomes. This contributes to the genetic variability of progeny.

Law of Dominance (the "Third Law")

Mendel's Law of Dominance states that recessive alleles will always be masked by dominant alleles. Therefore, a cross between a homozygous dominant and a homozygous recessive will always express the dominant phenotype, while still having a heterozygous genotype. The Law of Dominance can be explained easily with the help of a mono hybrid cross experiment:- In a cross between two organisms pure for any pair (or pairs) of contrasting traits (characters), the character that appears in the F1 generation is called "dominant" and the one which is suppressed (not expressed) is called "recessive." Each character is controlled by a pair of dissimilar factors. Only one of the characters expresses. The one which expresses in the F1 generation is called Dominant. However, the law of dominance is not universally applicable.

Mendelian trait

A Mendelian trait is one that is controlled by a single locus in an inheritance pattern. In such cases, a mutation in a single gene can cause a disease that is inherited according to Mendel's laws. Examples include sickle-cell anemia, Tay–Sachs disease, cystic fibrosis and xeroderma pigmentosa. A disease controlled by a single gene contrasts with a multi-factorial disease, like heart disease, which is affected by several loci (and the environment) as well as those diseases inherited in a non-Mendelian fashion.

Non-Mendelian inheritance

In four o'clock plants, the alleles for red and white flowers show incomplete dominance. As seen in the F₁ generation, heterozygous (wr) plants have "pink" flowers—a mix of "red" (rr) and "white" (ww) coloring. The F₂ generation shows a 1:2:1 ratio of red:pink:white

Mendel explained inheritance in terms of discrete factors—genes—that are passed along from generation to generation according to the rules of probability. Mendel's laws are valid for all sexually reproducing organisms, including garden peas and human beings. However, Mendel's laws stop short of explaining some patterns of genetic inheritance. For most sexually reproducing organisms, cases where Mendel's laws can strictly account for the patterns of inheritance are relatively rare. Often, the inheritance patterns are more complex. 

The F₁ offspring of Mendel's pea crosses always looked like one of the two parental varieties. In this situation of "complete dominance," the dominant allele had the same phenotypic effect whether present in one or two copies. But for some characteristics, the F₁ hybrids have an appearance in between the phenotypes of the two parental varieties. A cross between two four o'clock (Mirabilis jalapa) plants shows this common exception to Mendel's principles. Some alleles are neither dominant nor recessive. The F₁ generation produced by a cross between red-flowered (RR) and white flowered (WW) Mirabilis jalapa plants consists of pink-colored flowers. Neither allele is dominant in this case. This third phenotype results from flowers of the heterzygote having less red pigment than the red homozygotes. Cases in which one allele is not completely dominant over another are called incomplete dominance. In incomplete dominance, the heterozygous phenotype lies somewhere between the two homozygous phenotypes.

A similar situation arises from codominance, in which the phenotypes produced by both alleles are clearly expressed. For example, in certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens have a color described as "erminette", speckled with black and white feathers. Unlike the blending of red and white colors in heterozygous four o'clocks, black and white colors appear separately in chickens. Many human genes, including one for a protein that controls cholesterol levels in the blood, show codominance, too. People with the heterozygous form of this gene produce two different forms of the protein, each with a different effect on cholesterol levels.

In Mendelian inheritance, genes have only two alleles, such as a and A. In nature, such genes exist in several different forms and are therefore said to have multiple alleles. A gene with more than two alleles is said to have multiple alleles. An individual, of course, usually has only two copies of each gene, but many different alleles are often found within a population. One of the best-known examples is coat color in rabbits. A rabbit's coat color is determined by a single gene that has at least four different alleles. The four known alleles display a pattern of simple dominance that can produce four coat colors. Many other genes have multiple alleles, including the human genes for ABO blood type.

Furthermore, many traits are produced by the interaction of several genes. Traits controlled by two or more genes are said to be polygenic traits. Polygenic means "many genes." For example, at least three genes are involved in making the reddish-brown pigment in the eyes of fruit flies. Polygenic traits often show a wide range of phenotypes. The broad variety of skin color in humans comes about partly because at least four different genes probably control this trait.