Editing Genotype (section)

== Mendelian inheritance ==
{{Main|Mendelian inheritance}}[[Image:Punnett square mendel flowers.svg|thumb|right|Here the relation between genotype and phenotype is illustrated, using a [[Punnett square]], for the character of petal colour in a [[pea plant]]. The letters B and b represent alleles for colour and the pictures show the resultant flowers. The diagram shows the cross between two heterozygous parents where B represents the dominant allele (purple) and b represents the recessive allele (white).]]Traits that are determined exclusively by genotype are typically inherited in a [[Mendelian inheritance|Mendelian]] pattern. These laws of inheritance were described extensively by [[Gregor Mendel]], who performed experiments with pea plants to determine how traits were passed on from generation to generation.<ref name=":0">{{Cite web|title=Gregor Mendel and the Principles of Inheritance {{!}} Learn Science at Scitable|url=http://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593|access-date=2021-11-15|website=www.nature.com|language=en}}</ref> He studied phenotypes that were easily observed, such as plant height, petal color, or seed shape.<ref name=":0" /> He was able to observe that if he crossed two true-breeding plants with distinct phenotypes, all the offspring would have the same phenotype. For example, when he crossed a tall plant with a short plant, all the resulting plants would be tall. However, when he self-fertilized the plants that resulted, about 1/4 of the second generation would be short. He concluded that some traits were [[Dominance (genetics)|dominant]], such as tall height, and others were recessive, like short height. Though Mendel was not aware at the time, each phenotype he studied was controlled by a single gene with two alleles. In the case of plant height, one allele caused the plants to be tall, and the other caused plants to be short. When the tall allele was present, the plant would be tall, even if the plant was heterozygous. In order for the plant to be short, it had to be homozygous for the recessive allele.<ref name=":0" /><ref>{{Cite web|title=12.1 Mendel's Experiments and the Laws of Probability – Biology {{!}} OpenStax|url=https://openstax.org/books/biology/pages/12-1-mendels-experiments-and-the-laws-of-probability|access-date=2021-11-15|website=openstax.org|language=en}}</ref>

One way this can be illustrated is using a [[Punnett square]]. In a Punnett square, the genotypes of the parents are placed on the outside. An uppercase letter is typically used to represent the dominant allele, and a lowercase letter is used to represent the recessive allele. The possible genotypes of the offspring can then be determined by combining the parent genotypes.<ref>{{Cite web|date=2016-09-21|title=3.6: Punnett Squares|url=https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Introductory_Biology_(CK-12)/03%3A_Genetics/3.06%3A_Punnett_Squares|access-date=2021-11-15|website=Biology LibreTexts|language=en}}</ref> In the example on the right, both parents are heterozygous, with a genotype of Bb. The offspring can inherit a dominant allele from each parent, making them homozygous with a genotype of BB. The offspring can inherit a dominant allele from one parent and a recessive allele from the other parent, making them heterozygous with a genotype of Bb. Finally, the offspring could inherit a recessive allele from each parent, making them homozygous with a genotype of bb. Plants with the BB and Bb genotypes will look the same, since the B allele is dominant. The plant with the bb genotype will have the recessive trait.

These inheritance patterns can also be applied to [[Genetic disorder|hereditary diseases]] or conditions in humans or animals.<ref name=":12">{{Cite book|last1=Alliance|first1=Genetic|url=https://www.ncbi.nlm.nih.gov/books/NBK132145/|title=Classic Mendelian Genetics (Patterns of Inheritance)|last2=Health|first2=District of Columbia Department of|date=2010-02-17|publisher=Genetic Alliance|language=en}}</ref><ref name=":2">{{Cite web|title=Mendelian Inheritance|url=https://www.genome.gov/genetics-glossary/Mendelian-Inheritance|access-date=2021-11-15|website=Genome.gov|language=en}}</ref><ref name=":3">{{Cite book|last=Strachan|first=T.|url=https://www.worldcat.org/oclc/1083018958|title=Human molecular genetics|publisher=[[Garland Science]]|others=Andrew P. Read|year=2018|isbn=978-0-429-82747-1|edition=5th|location=New York|oclc=1083018958}}</ref> Some conditions are inherited in an [[Autosome|autosomal]] dominant pattern, meaning individuals with the condition typically have an affected parent as well. A classic pedigree for an autosomal dominant condition shows affected individuals in every generation.<ref name=":12"/><ref name=":2" /><ref name=":3" /> [[File:Example autosomal dominant pedigree 01.png|thumb|An example of a pedigree for an autosomal dominant condition]] Other conditions are inherited in an autosomal recessive pattern, where affected individuals do not typically have an affected parent. Since each parent must have a copy of the recessive allele in order to have an affected offspring, the parents are referred to as carriers of the condition.<ref name=":12"/><ref name=":2" /><ref name=":3" />

In autosomal conditions, the sex of the offspring does not play a role in their risk of being affected. In sex-linked conditions, the sex of the offspring affects their chances of having the condition. In humans, females inherit two [[X chromosome]]s, one from each parent, while males inherit an X chromosome from their mother and a Y chromosome from their father. X-linked dominant conditions can be distinguished from autosomal dominant conditions in pedigrees by the lack of transmission from fathers to sons, since affected fathers only pass their X chromosome to their daughters.<ref name=":3" /><ref name=":12"/><ref name=":4">{{Cite web|date=2020-06-24|title=4.4.1: Inheritance patterns for X-linked and Y-linked genes|url=https://bio.libretexts.org/Courses/University_of_Arkansas_Little_Rock/Genetics_BIOL3300_(Fall_2021)/Genetics_Textbook/04%3A_Inheritance/4.04%3A_Exceptions_to_autosomal_inheritance/4.4.01%3A_Inheritance_patterns_for_X-linked_and_Y-linked_genes|access-date=2021-11-15|website=Biology LibreTexts|language=en}}</ref> In X-linked recessive conditions, males are typically affected more commonly because they are hemizygous, with only one X chromosome. In females, the presence of a second X chromosome will prevent the condition from appearing. Females are therefore carriers of the condition and can pass the trait on to their sons.<ref name=":3" /><ref name=":12" /><ref name=":4" />[[File:Example autosomal recessive pedigree.png|thumb|An example of a pedigree for an autosomal recessive condition]]

Mendelian patterns of inheritance can be complicated by additional factors. Some diseases show incomplete [[penetrance]], meaning not all individuals with the disease-causing allele develop signs or symptoms of the disease.<ref name=":3" /><ref name=":8">{{Cite web|date=2021-01-13|title=14.2: Penetrance and Expressivity|url=https://bio.libretexts.org/Courses/Lumen_Learning/Book-_Biology_for_Majors_I_(Lumen)/14%3A_Module_12-_Trait_Inheritance/14.02%3A_Penetrance_and_Expressivity|access-date=2021-11-19|website=Biology LibreTexts|language=en}}</ref><ref name=":9">{{Cite web|title=Phenotype Variability: Penetrance and Expressivity {{!}} Learn Science at Scitable|url=http://www.nature.com/scitable/topicpage/phenotype-variability-penetrance-and-expressivity-573|access-date=2021-11-19|website=www.nature.com|language=en}}</ref> Penetrance can also be age-dependent, meaning signs or symptoms of disease are not visible until later in life. For example, [[Huntington's disease|Huntington disease]] is an autosomal dominant condition, but up to 25% of individuals with the affected genotype will not develop symptoms until after age 50.<ref>{{Citation|last1=Caron|first1=Nicholas S.|title=Huntington Disease|date=1993|url=http://www.ncbi.nlm.nih.gov/books/NBK1305/|work=GeneReviews®|editor-last=Adam|editor-first=Margaret P.|place=Seattle (WA)|publisher=University of Washington, Seattle|pmid=20301482|access-date=2021-11-19|last2=Wright|first2=Galen EB|last3=Hayden|first3=Michael R.|editor2-last=Ardinger|editor2-first=Holly H.|editor3-last=Pagon|editor3-first=Roberta A.|editor4-last=Wallace|editor4-first=Stephanie E.}}</ref> Another factor that can complicate Mendelian inheritance patterns is variable [[Expressivity (genetics)|expressivity]], in which individuals with the same genotype show different signs or symptoms of disease.<ref name=":3" /><ref name=":8" /><ref name=":9" /> For example, individuals with [[polydactyly]] can have a variable number of extra digits.<ref name=":8" /><ref name=":9" />