Frequencies of alleles and genotypes
The information we collect provides the numbers of each genotype at a
locus. This is illustrated for an egg-white protein locus in Scottish eider
ducks (Table 4.1). This species was severely reduced due to harvest for
feathers for pillows and eiderdowns. Genotype frequencies are simply
calculated from the proportion of the total sample of that type (e.g.
genotype frequency of FF37/670.552).
For comparative and predictive use, the information is usually
reported in the form of allele frequencies (often referred to as gene frequencies), rather than genotype frequencies. We will use the lettersp
andqto represent the allele frequencies for two alleles at a locus. The
frequency of the F allele (p) in the eider duck population is simply the
proportion of all alleles examined which are F. Note that we double
the number of each homozygote, and the total, as the ducks are
diploid (each bird has inherited one copy of the locus from each of its
parents).