Heritability and additive genetic variation

Side Box 2.2

Additive Genetic Variation versus Dominance Variation

What is the source of heritable variation? The resemblance between parents and offspring is entirely due to the additive effect that genes have on phenotype. Not all genetic variation is additive. For example, dominance variation arises from the degree to which one allele at a locus alters the effect of the allele on the complementary chromosome. A useful way to think about additive genetic variation is to consider the proportion of the phenotype that can be predicted from a knowledge of the number of A versus a allele are present in the genotype -- or the genotypic value. Consider an aa individual to have zero A alleles, Aa has one A allele, and AA has two A alleles. Only in the idealized case when alleles at a genetic locus are co-dominant, are the genes are said to act in a purely additive fashion. Dominance reduces the additive genetic variation, while at the same time increasing the dominance variation.

Co-dominant alleles

In the case of a co-dominant set of alleles, knowlege of genotype allows us to perfectly predict the value of the phenotype. A regression (red line) of phenotypic value on number of A alleles, genotypic value, yields a perfect predictive line.

Dominant and recessive alleles

In the case of a dominant A allele, knowlege of genotypic value allows us to only imperfectly predict the value of the phenotype. Because heterozgotes and dominant homozygotes have the same phenotypic value, the fit between the regression (red line) of phenotypic value on genotypic value has error (black arrows). There is still a significant regression slope so some of the variation is due to additive genetic causes (red line) and some is due to dominance variation (black arrows).

Overdominance

In the case of overdominance, knowledge of genotype does not allow us to predict the value of the phenotype with any confidence. There is no slope to the regression line so none of the variation is due to "additive genetic effects" of alleles. All variation in our model is due to error in our model which arises from dominance variation (black arrows). Consider two parents that are homozygous for alternative alleles (aa and AA). If behavior were strongly determined by an overdominant allele then the heterozygous progeny would not at all resemble the homozygous parents.

Underdominance

Likewise, in the case of underdominance, knowledge of genotype does not allow us to predict the value of the phenotype with any confidence. There is no slope to the regression line so none of the variation is due to "additive genetic effects" of alleles. All variation in our model is due to error in our model which arises from dominance variation (black arrows).

Back to 2. Genes and Behavior

	Co-dominant alleles In the case of a co-dominant set of alleles, knowlege of genotype allows us to perfectly predict the value of the phenotype. A regression (red line) of phenotypic value on number of A alleles, genotypic value, yields a perfect predictive line.
	Dominant and recessive alleles In the case of a dominant A allele, knowlege of genotypic value allows us to only imperfectly predict the value of the phenotype. Because heterozgotes and dominant homozygotes have the same phenotypic value, the fit between the regression (red line) of phenotypic value on genotypic value has error (black arrows). There is still a significant regression slope so some of the variation is due to additive genetic causes (red line) and some is due to dominance variation (black arrows).
	Overdominance In the case of overdominance, knowledge of genotype does not allow us to predict the value of the phenotype with any confidence. There is no slope to the regression line so none of the variation is due to "additive genetic effects" of alleles. All variation in our model is due to error in our model which arises from dominance variation (black arrows). Consider two parents that are homozygous for alternative alleles (aa and AA). If behavior were strongly determined by an overdominant allele then the heterozygous progeny would not at all resemble the homozygous parents.
	Underdominance Likewise, in the case of underdominance, knowledge of genotype does not allow us to predict the value of the phenotype with any confidence. There is no slope to the regression line so none of the variation is due to "additive genetic effects" of alleles. All variation in our model is due to error in our model which arises from dominance variation (black arrows).