readings - introduction - exercise instructions - homework - instructor hints
Note: This project was originally conducted as part of a study by Fred Nijhout and Susan Paulsen at
(PLEASE Email jvondras [at] mbc [dot] edu if you use this for a class)
Brakefield, P.M. et al. 1996. Development, plasticity and evolution of butterfly eyespot
patterns. Nature. 384:236-242.
Nijhout, H.F. 1996. Focus on butterfly eyespot development. Nature. 384: 209-210.
Quantitative traits: do not follow simple Mendelian dominance patterns, but alleles interact to produce a range of phenotypes.
Today we will study how evolutionary biologists analyze selection for characters that are affected by a multitude of loci (continuous traits). Today's goal is to illustrate the mechanism for predicting the response to selection when there are many genes affecting the character on which selection is occurring . This subject falls under the broad heading of Quantitative Genetics
Heritability is the proportion of phenotypic variance that is additive. If a character has no additive genetic variance in a population, it will not be inherited from parent to offspring. Your text describes the following intuitive explanation of heritability.
"Consider a parent that differs from the population by a certain amount. If its offspring also deviate by the same amount, heritability is 1.0; if the offspring have the same mean as the population, the heritability is 0.0; if the offspring deviate from the mean in the same direction as their parents but to a lesser extent, heritability is between 0.0 and 1.0. "
Because biologists often don’t know what alleles an
individual has, or how those alleles contribute to phenotype, an indirect
method of estimating additive genetic variance is used.
We will take the measurements necessary to calculate the heritability of characters in the wing pattern of the buckeye butterfly, Precis coenia .
For two variables x (parental value) and y (average value for the offspring of that parent), the regression coefficient (bxy ) is defined as the ratio:
where covxy is the covariance between x and y, and measures the degree to which parents and offspring "covary", and var x = the variance of the parental measure x.
Use the formulas below to calculate the variances and covariances.
Variance is the average squared deviation from the population mean
Covariance is the product of the deviation from the mean of two different variables (x and y)
A similar project to measure heritability in parent/offspring regression could be done in conjunction with an artificial selection lab on Brassica or Drosophila, for example.