Chapter 7 cont.

III) Selection and Quantitative Traits

A) Definition and Background:

Most phenotypic traits are not single gene traits, but are actually multiple gene products working together.

1) Metric Characters – phenotypic characters that vary in a continuously way (i.e., 1.021, 1.0212, etc.)(e.g., weight, height, length, etc)

2) Meristic Characters – phenotypic characters that vary discretely (i.e., 1, 2, 3, etc.)(e.g., number of hairs, number of whorls, etc.)

3) Quantitative Traits – continuously varying characteristics with polygenetic inheritance.

4) Heritability – the fraction of observed or measured variation in a trait that is due to genes.

5) Measure of Variance – the degree to which the population as a whole deviates from the mean

V =

where n is the number of individuals

or:

V =

6) Phenotypic variation – (V_P) total observed or measured variation in a trait.

7) Environmental variation (V_E) is the amount of variation due to the environment. AA in good environment is big but in bad environment is small.

8) Heritability – is the fraction of the phenotypic that is due to genetic variation (V_G).

Heritability =

9) Narrow Sense Heritability – h² the fraction of the phenotypic variation that is due to additive genetic variation (V_A) and not dominance variation (V_D)

V_G = V_A + V_D

h² =

Assuming that one A allele changes the phenotype by 0.5 units and there is no dominance

Assuming that there is complete dominance

B) Response to Selection – the response to selection must be a combination of the degree to which the character is heritable and the strength of selection

R = h² S or h² =

In Truncating Selection:

S = R = h² =