The extent of genetic diversity at a locus is expressed as heterozygosity.
Observed heterozygosity (H0
) is simply the number of heterozygotes at
a locus divided by the total number of individuals sampled. For
example, the observed frequency of heterozygotes at the egg-white
protein locus in eider ducks (Table 4.1) is 24 / 670.36. A similar procedure is followed when there are more than two alleles at a locus. For the
Laysan finch microsatellites (Table 4.2), the observed heterozygosity
is [10811] / 440.659. Generally, expected heterozygosity (He
),
described later, is reported for outbreeding species, as it is less sensitive
to sample size than observed heterozygosity.
Having described the parameters used to characterize the genetic
composition of populations, we will now deal with factors that influence the frequencies of alleles and genotypes. The remainder of this
chapter covers the consequences of different mating systems. Chapters
6 and 7 deal with mutation, migration and selection and Chapter 8 with
the consequences of small population size.
We now address the questions, what determines the frequencies of
genotypes in a population? What is the relationship between allele and
genotype frequencies under random mating