Heritability – “the fraction of the total variation in a trait that is due to variation in genes.” (Freeman and Herron, 2007)

Vp – the total variance in the phenotypic trait of a population.

Typically, this variation is considered as the phenotypic variation.

VP = VG + VE

VG = Genetic variationVE = Environmental variation

Broad Sense Heritability h2 or H2

The problem? VG represents all of the genetic variation as a single value.

Real genetic variation is complicated.

VA = additive genetic varianceVD = dominance genetic varianceVI = variance due to epistatic interactions

Additive Variation –VA

• 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.

Additive Variation –VA

A1 = 0.5, A2 = 0.5

Note: The best fit line (far right) explains all of the genetic variation. VG = VA.

Dominance Variation –VD

• 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.

Dominance Variation –VD

A1 = 0.5, A2 = 0.5

Note: Adding a second copy of A2 does not change the phenotype. The Dominance Variation (VD) does not explain all of the Genetic Variation (VG). VG = VA + VD

Epistatic Effects –VI

• 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 h2 or H2

In determining heritability, only narrow-sense heritability is used, because only the variation due to additive effects permits predictions due to selection.