Introduction

Remember that long list of assumptions associated with derivation of the Hardy-Weinberg principle that I went over a couple of lectures ago? Well, we're about to begin violating assumptions to explore the consequences, but we're not going to violate them in order. We're first going to violate Assumption #2:

Genotypes mate at random with respect to their genotype at this particular locus.

There are many ways in which this assumption might be violated:

• Some genotypes may be more successful in mating than others - sexual selection.

• Genotypes that are different from one another may mate more often than expected - disassortative mating, e.g., self-incompatibility alleles in flowering plants, MHC loci in humans (the smelly t-shirt experiment) [2].

• Genotypes that are similar to one another may mate more often than expected - assortative mating.

• Some fraction of the offspring produced may be produced asexually.

• Individuals may mate with relatives - inbreeding.
  • self-fertilization

  • sib-mating

  • first-cousin mating

  • parent-offspring mating

  • etc.

When there is sexual selection or disassortative mating genotypes differ in their chances of being included in the breeding population. As a result, allele and genotype frequencies will tend to change from one generation to the next. We'll talk a little about these types of departures from random mating when we discuss the genetics of natural selection in a few weeks, but we'll ignore them for now. In fact, we'll also ignore assortative mating, since it's properties are fairly similar to those of inbreeding, and inbreeding is easier to understand.