Back to 2. Genes and Behavior

Side Box 2.6. Additive versus Dominance Variation

Relatives vary in the proportion of alleles that they share in common, and the coefficient of relatedness is a measure of the probability that an allele is in common between relatives. For example, the coefficient of relatedness is 0.5 for one parent and offspring, 0.5 for sibs, 0.25 for half sibs, and 1.0 for dizygotic twins. By sharing alleles, relatives end up sharing the additive effects of individual alleles.

While the relationship for the proportion of shared alleles between a parent and an offspring is exact, the relationship between sibs is probabilistic. Offspring get exactly one half of their genes from a single parent. However, it is theoretically possible that full sibs could share none of their alleles, one of each pair of alleles, or both pairs of the alles. How could this be? First we can label alleles that parents could potentially give to their children: 1, 2 from their mother and 3, 4 from their father. Likewise, the situation where sibs share no genes occurs with probability one-quarter for any given gene. The most likely situation is when sibs share a single allele of a pair of alleles, which occurs with probability of one-half. Finally, if offspring happen to share both copies of their alleles in common they also share any dominance relations between the two pairs of alleles that they inherit. This occurs with probability one quarter for any given gene.

If sibs happen to share both pairs of alleles 1 and 4, and 1 happens to be dominant to 4, then the sibs still inherit the additive effect of the genes as well as the dominance variation. Sibs still resemble each other largely because they share the additive effects of genes. However, sharing a similar dominance configuration also increases the resemblance between sibs relative to the resemblance between parents and offspring. Sibs resemble each other more so than they do their parents. This additional genetic component of variance is why some sibs appear to be identical twins. The additional dominance variation that sibs share (no other relationship shares this dominance variation) is also shared by twins. While full sibs share dominance variation, offspring do not share dominance variation with their parents, only additive genetic variance.