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Progeny

1/4 BB, 1/2 Bb, 1/4 bb

1/4(3) + 1/2(3) + 1/4(1)

Table 9.3 Examples of parental and progeny mean phenotypes that illustrate the impacts of additive gene action (top) or complete dominance gene action (bottom). For both types of gene action, the phenotypic value of each genotype is given and the genotypes of two possible parental crosses are shown along with the genotypes in the progeny from each cross. Under additive gene action the mean phenotypic values are identical in the parents and progeny because phenotypic values are a function of allele frequencies and alleles are identical in parents and progeny. In contrast, under complete dominance parent and progeny mean phenotypic values differ because phenotypic values are a function of the genotype and genotype frequencies differ between parents and progeny.

of two identical alleles (homozygous genotypes) or two dissimilar alleles (heterozygous genotypes).

Two examples of the average phenotypic resemblance between parents and offspring under complete dominance are shown in the bottom half of Table 9.3. When crossing BB x bb parents (a) to yield a population of Bb progeny, the parental mean phenotype and the progeny mean phenotype are not identical. This lack of parent-offspring phenotypic resemblance occurs because the parental population and the progeny population do not share any genotypes in common. The parents are both homozygotes while all progeny are heterozygotes. The mean phenotype for parents and progeny is closer for the Bb x Bb parental cross (b). This occurs because 50% of the progeny have an identical genotype to the parents. Thus, with dominance, shared genotype frequencies will lead to phenotypic resemblance. Because par-ticulate inheritance breaks up genotypes, alleles are inherited but diploid genotypes are not. Genotype frequencies are a consequence of how gametes combine to make progeny genotypes. Thus, the genotype effects of dominance (VD) do not contribute to average phenotypic resemblance between parents and offspring. Like dominance, epistasis (Vj) is also a property of the genotype that does not contribute to average phenotypic resemblance between parents and offspring because genotypes are not inherited. An exception is that additive by additive epistasis does contribute to resemblance between parents and offspring.

Whereas VD does not contribute to the resemblance of parents and offspring, dominance variance can cause phenotypic resemblance between other types of relatives. In particular, dominance variance contributes to the phenotypic resemblance among full siblings (brothers and sisters). This occurs because full siblings can inherit identical genotypes since they share the same two parents in common. Thus, a shared heterozygote genotype would cause two full siblings to share a genotypic value caused by dominance. Resemblance among relatives is explored more fully in section 10.6.

Genotype-by-environment interaction (VGxE)

Phenotypic variation can be caused by the combination of genotypes and environments in a population. Up to this point we have assumed that genotypes are all equally sensitive to their environments, meaning that a change of environment would impact the phenotype of all genotypes to the same extent. In fact, genotypes very often have different degrees of sensitivity to environmental conditions. This cause of phenotypic variance is called genotype-by-environment interaction and is symbolized by VGxE. This adds another term to the expression for the independent causes of total phenotypic variation in a population:

Genotype-by-environment interaction

(VCxE) The contribution to total phenotypic variation caused by genotypes that vary in their sensitivity to different environments. Also known as phenotypic plasticity.

In one form of genotype-by-environment interaction, genotypes are extremely sensitive to changes in the environment such that the total phenotypic variance changes markedly between two or more environments. In another form of genotype-by-environment interaction, genotypes change phenotypic rank in different environments. For example, genotype AA has a larger phenotypic value than genotype Aa in environment one but in environment two the order is reversed with genotype Aa having the larger phenotypic value.

Hypothetical genotypic ( VG), environmental (VE), and genotype-by-environment interaction (VGxE) contributions to total phenotypic variation are illustrated in Fig. 9.6. The figure illustrates the results of an imaginary experiment where individuals of four different genotypes are subjected to two environments. Lines connect the phenotype measured for the same genotype in the two environments. Genotypic variation only (VG; Fig. 9.6a) means that the four genotypes have different phenotypes but that no genotype changes its phenotype between the environments (notice that the spread of phenotypes does not change

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