Editing Genotype (section)

== Phenotype ==
{{Main|Phenotype}}

Any given gene will usually cause an observable change in an organism, known as the phenotype.  The terms [[genotype–phenotype distinction|genotype and phenotype]] are distinct for at least two reasons:
* To distinguish the source of an observer's knowledge (one can know about genotype by observing DNA; one can know about phenotype by observing outward appearance of an organism).
* Genotype and phenotype are not always directly correlated.  Some genes only express a given phenotype in certain environmental conditions.  Conversely, some phenotypes could be the result of multiple genotypes. The genotype is commonly mixed up with the phenotype which describes the result of both the genetic and the environmental factors giving the observed expression (e.g. blue eyes, hair color, or various hereditary diseases).

A simple example to illustrate genotype as distinct from phenotype is the flower colour in pea plants (see [[Gregor Mendel]]). There are three available genotypes, PP ([[homozygous dominant]]), Pp (heterozygous), and pp (homozygous recessive). All three have different genotypes but the first two have the same phenotype (purple) as distinct from the third (white).

A more technical example to illustrate genotype is the [[single-nucleotide polymorphism]] or SNP.  A SNP occurs when corresponding sequences of [[DNA]] from different individuals differ at one DNA base, for example where the sequence AAGCCTA changes to AAGCTTA.<ref>Vallente, R. U., PhD. (2020). Single Nucleotide Polymorphism. ''Salem Press Encyclopedia of Science''.</ref> This contains two alleles : C and T. SNPs typically have three genotypes, denoted generically AA Aa and aa. In the example above, the three genotypes would be CC, CT and TT. Other types of [[genetic marker]], such as [[microsatellite]]s, can have more than two alleles, and thus many different genotypes.

Penetrance is the proportion of individuals showing a specified genotype in their phenotype under a given set of environmental conditions.<ref>{{cite dictionary |title=A dictionary of zoology |date=2009 |publisher=Oxford University Press |editor=Allaby, Michael |isbn=9780199233410 |edition=3rd |location=Oxford |oclc=260204631}}</ref>