In biological populations, a wide range of values have been observed for the fixation index (Table 2.9). Fixation indices have frequently been estimated with allozyme data (see Box 2.2). Estimates of
The fixation index can be understood as a measure of the correlation between the states of the two alleles in a diploid genotype. When F = 0 there is no correlation between the two alleles in a genotype, the states of the two alleles are independent as we expect under Mendel's first law. If F > 0 there is a positive correlation such that if one of the alleles in a genotype is an A, for example, then the other allele will have a correlated state and also be an A. When F < 0 there is a negative correlation between the states of the two alleles in a genotype and heterozygotes are more common since the two alleles tend to have different states.
Extending the fixation index to loci with more than two alleles requires a means to calculate the expected frequency of genotypes with identical alleles (or with non‐identical alleles) for an arbitrary number of alleles at one diploid locus. This can be accomplished by adding up all of the expected frequencies of each possible homozygous genotype and subtracting this total from 1 or summing the expected frequencies of all heterozygous genotypes:
Table 2.9 Estimates of the fixation index (
| Species | Mating system |
|
Method | References |
|---|---|---|---|---|
| Humans | ||||
| Homo sapiens | outcrossed | 0.0001–0.046 | pedigree | Jorde (1997) |
| Snail | ||||
| Bulinus truncates | selfed & outcrossed | 0.6–1.0 | microsatellites | Viard et al. (1997) |
| Domestic dogs | ||||
| Breeds combined | outcrossed | 0.33 | allozyme | Christensen et al. (1985) |
| German Shepard | outcrossed | 0.10 | ||
| Mongrels | outcrossed | 0.06 | ||
| Plants | ||||
| Arabidopsis thaliana | Selfed | 0.99 | allozyme | Abbott et al. (1989) |
| Pinus ponderosa | outcrossed | −0.37 | allozyme | Brown (1979) |
Box 2.2 Protein locus or allozyme genotyping
Determining the genotypes of individuals at enzymatic protein loci is a rapid technique to estimate genotype frequencies in populations. Protein analysis was the primary molecular genotyping technique for several decades before DNA‐based techniques became widely available. Alleles at loci that code for proteins with enzymatic function can be ascertained in a multi‐step process. First, fresh tissue samples are ground up under conditions that preserve the function of proteins. Next, these protein extracts are loaded onto starch gels and exposed to an electric field. The electrical current results in electrophoresis where proteins are separated based on their ratio of molecular charge to molecular weight. Once electrophoresis is complete, the gel is then “stained” to visualize specific enzymes. The primary biochemical products of protein enzymes are not themselves visible. However, a series of biochemical reactions in a process called enzymatic coupling can be used to eventually produce a visible product (often nitro blue tetrazolium or NBT) at the site where the enzyme is active (see Figure 2.11). If different DNA sequences at a protein enzyme locus result in different amino acid sequences that differ in net charge, then multiple alleles will appear in the gel after staining.
