Editing Genotype (section)

== Non-Mendelian inheritance ==
{{Main|Non-Mendelian inheritance}}

Many traits are not inherited in a Mendelian fashion, but have more complex patterns of inheritance.

=== Incomplete dominance ===
For some traits, neither allele is completely dominant. Heterozygotes often have an appearance somewhere in between those of homozygotes.<ref name=":52">{{Cite web|title=Genetic Dominance: Genotype-Phenotype Relationships {{!}} Learn Science at Scitable|url=http://www.nature.com/scitable/topicpage/genetic-dominance-genotype-phenotype-relationships-489|access-date=2021-11-15|website=www.nature.com|language=en}}</ref><ref name=":6">{{Citation|last=Frizzell|first=M.A.|title=Incomplete Dominance|date=2013|url=https://linkinghub.elsevier.com/retrieve/pii/B9780123749840007841|encyclopedia=Brenner's Encyclopedia of Genetics|pages=58–60|publisher=Elsevier|language=en|doi=10.1016/b978-0-12-374984-0.00784-1|isbn=978-0-08-096156-9|access-date=2021-11-15}}</ref> For example, a cross between true-breeding red and white ''[[Mirabilis jalapa]]'' results in pink flowers.<ref name=":6" />

=== Codominance ===
Codominance refers to traits in which both alleles are expressed in the offspring in approximately equal amounts.<ref name=":7">{{Citation|last=Xia|first=X.|title=Codominance|date=2013|url=https://linkinghub.elsevier.com/retrieve/pii/B9780123749840002783|encyclopedia=Brenner's Encyclopedia of Genetics|pages=63–64|publisher=Elsevier|language=en|doi=10.1016/b978-0-12-374984-0.00278-3|isbn=978-0-08-096156-9|access-date=2021-11-15}}</ref> A classic example is the [[ABO blood group system]] in humans, where both the A and B alleles are expressed when they are present. Individuals with the AB genotype have both A and B proteins expressed on their red blood cells.<ref name=":7" /><ref name=":52"/>

=== Epistasis ===
{{Main|Epistasis}}

Epistasis is when the phenotype of one gene is affected by one or more other genes.<ref>{{Cite journal|last1=Gros|first1=Pierre-Alexis|last2=Nagard|first2=Hervé Le|last3=Tenaillon|first3=Olivier|date=2009-05-01|title=The Evolution of Epistasis and Its Links With Genetic Robustness, Complexity and Drift in a Phenotypic Model of Adaptation|url=https://www.genetics.org/content/182/1/277|journal=Genetics|language=en|volume=182|issue=1|pages=277–293|doi=10.1534/genetics.108.099127|issn=0016-6731|pmc=2674823|pmid=19279327}}</ref> This is often through some sort of masking effect of one gene on the other.<ref>{{Cite book|last=Rieger, Rigomar.|url=https://www.worldcat.org/oclc/2202589|title=Glossary of genetics and cytogenetics : classical and molecular|date=1976|publisher=Springer-Verlag|others=Michaelis, Arnd,, Green, Melvin M.|isbn=0-387-07668-9|edition=4th completely rev. |location=Berlin|oclc=2202589}}</ref> For example, the "A" gene codes for hair color, a dominant "A" allele codes for brown hair, and a recessive "a" allele codes for blonde hair, but a separate "B" gene controls hair growth, and a recessive "b" allele causes baldness. If the individual has the BB or Bb genotype, then they produce hair and the hair color phenotype can be observed, but if the individual has a bb genotype, then the person is bald which masks the A gene entirely.

=== Polygenic traits ===
{{Main|Polygene}}

A polygenic trait is one whose phenotype is dependent on the additive effects of multiple genes. The contributions of each of these genes are typically small and add up to a final phenotype with a large amount of variation. A well studied example of this is the number of sensory bristles on a fly.<ref>{{Cite journal|last=Mackay|first=T. F.|date=December 1995|title=The genetic basis of quantitative variation: numbers of sensory bristles of Drosophila melanogaster as a model system|url=https://www.ncbi.nlm.nih.gov/pubmed/8533161|journal=Trends in Genetics|volume=11|issue=12|pages=464–470|doi=10.1016/s0168-9525(00)89154-4|issn=0168-9525|pmid=8533161}}</ref> These types of additive effects is also the explanation for the amount of variation in human eye color.