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title: "2.5 The expression of genes, of their alleles, in the phenotype"
canonical: "https://wiki.groenkennisnet.nl/space/TAB/270893234/2.5%20The%20expression%20of%20genes%2C%20of%20their%20alleles%2C%20in%20the%20phenotype"
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In all body cells the chromosomes are present in duplo: one originates from the sire and one from the dam. Therefore all genes are present in duplo. These genes might be identical: the allele originating from the sire is identical to the one from the dam. Then, an animal is homozygous for that gene. It implies that progeny of this animal always get that allele from this parent. The alleles originating from sire and dam might be different. Then an animal is heterozygous for that gene. This implies that progeny of this animal might get one of these two different alleles of this parent.

> ℹ️ ### Definition
> ℹ️ 
> ℹ️ ***Homozygote ****is an individual carrying two copies of the same allele at a locus, e.g. ee or EE.*
> ℹ️ 
> ℹ️ ***Heterozygote ****is an individual carrying two distinct alleles at a locus, e.g. Ee.*

For a certain gene we can distinguish three different genotypes: e.g. EE, Ee or ee. These combinations of alleles may cause different phenotypes. Suppose E is responsible for the production of the protein eumelanin in skin cells of dogs that gives a black pigmentation of the skin and e is responsible for the production of phaeomelanin in skin cells of dogs that gives a red pigmentation. It is obvious that EE animals will be black and ee animals will be red. But what will be the color of Ee dogs? It reveals that they are also black!. This phenomenon is called dominance: the expression of the e-allele in heterozygous is not expressed in their phenotype. The allele E is dominant over e or from a point of view from allele e: it is recessive to allele E.

> ℹ️ ### Definitions
> ℹ️ 
> ℹ️ *A ****dominant allele ****is an allele that has an effect on the phenotype not only when it is homozygous but also when it is heterozygous. When allele E is dominant over e, EE and Ee have the same phenotypic value.*
> ℹ️ 
> ℹ️ *A ****recessive ****allele is an allele that is only has an effect on the phenotype when it is homozygous. Therefore if allele e is recessive, ee yields a different phenotype from Ee and EE, which have the same phenotype. *

In case a gene is involved in the expression of a quantitative trait e.g. body weight of a mature goat the alleles might have a different expression leading to small differences in mature body weight. Take the following examples:

1. GG animals weigh 40 kg, Gg animals 38 kg and gg animals 36 kg. The body weight of the heterozygous animal is exactly the average of the two homozygous animals. The two alleles have an additive effect. We call it co-dominance.
2. GG animals weigh 40 kg, Gg animals 42 kg and gg animals 36 kg. The body weight of the heterozygous animal is higher than the average of the two homozygous animals and even higher than the value of the highest homozygous parent. In this case we call it overdominance.

> ℹ️ ### Definitions
> ℹ️ 
> ℹ️ ***Co-dominance ****is a situation in which a heterozygote shows the phenotypic effects of both alleles equally. See ‘Additivity’.*
> ℹ️ 
> ℹ️ ***Additivity**** is the assumption that each allele influencing a trait does so independently of the other allele present at that locus and all other alleles at all other loci, e.g. if alleles G and g are worth 1 and -1 respectively then additivity assumes GG is worth 2, Gg is worth 0, and gg is worth –2.*
> ℹ️ 
> ℹ️ ***Overdominance**** occurs when the heterozygote has a genotypic value more extreme than either parent. *

In addition to the effects of different alleles of a gene at a single locus, alleles of different genes at different loci might influence each other and that might be expressed in the phenotype of the trait they influence. Again we have two possibilities: the effects of different alleles at different loci are additive: the effect is the sum of the effect of the individual alleles. When these effects are non-additive, this is called epistasis.

> ℹ️ ### Definition
> ℹ️ 
> ℹ️ ***Epistasis ****is when loci are non-additive. The genotypic value of a locus on a trait depends upon the genotypes at other loci or**** ****a situation in which the differential phenotypic expression of a genotype depends on the genotype at another locus.**** ***

> Macro (children)