Heritability – “the fraction of the total variation in a trait that is due to variation in genes.” (Freeman and Herron, 2007) V p – the total variance in the phenotypic trait of a population. Typically, this variation is considered as the phenotypic variation. V P = V G + V E V G = Genetic variation V E = Environmental variation

Broad Sense Heritability h 2 or H 2

V A = additive genetic variance V D = dominance genetic variance V I = variance due to epistatic interactions

Important because it is why relatives resemble each other. Alleles act independently. The phenotype of an organism is the sum of the effect of each allele, hence additive. Additive alleles are not affected by the presence of other alleles. The effect of additive alleles allow biologists to follow evolution in a predictable way.

A 1 = 0.5, A 2 = 0.5 Note: The best fit line (far right) explains all of the genetic variation. V G = V A.

The addition of alleles is not additive. Dominance is one type of variation where alleles interact (between sister alleles on other chromosome). The effect of an allele depends upon what it is paired with. Because of this dependence, the outcome of dominance variation is not entirely predictable - it is context dependent. This context disappears every generation because of meiosis. The pairing of an allele with a sister allele on another chromosome in forming a zygote is unpredictable. Because of this, the effects of dominance variation change every generation, and are not predictable.

A 1 = 0.5, A 2 = 0.5 Note: Adding a second copy of A 2 does not change the phenotype. The Dominance Variation (V D ) does not explain all of the Genetic Variation (V G ). V G = V A + V D

The effect of an allele at a locus is dependent the presence of an allele at another locus. The phenotype is dependent upon the allele at one locus interacting with an allele at another locus. Not a predictable outcome. Allele X may affect the phenotype one way in the presence of allele A, and affect the phenotype another way in the presence of allele B. Because of this dependence, the outcome of epistasis is not entirely predictable - it is context dependent. This context disappears every generation because of meiosis. When chromosomes independently assort and recombine, the pairings of alleles change. Because of this, the effects of epistasis change every generation, and are not predictable.

Narrow-Sense Heritability h 2 or H 2