Measuring Genetic Variation: Heterozygosity and Population Structure
Variation within a population is summarised by how often individuals carry two different alleles, and variation between populations by how much of the total sits between rather than within groups. Those two numbers turn samples of sequence into a statement about how populations are connected.
What a learner can do afterwards
- Calculates expected heterozygosity from allele frequencies.
- Interprets a between-population index as gene flow or its absence.
- Explains why a sample from two mixed populations looks short of heterozygotes.
1 · Read
Variation within a population is summarised by expected heterozygosity: the chance that two gene copies picked at random differ. With allele frequencies p and q it equals 2 times p times q. For p equals 0.3 and q equals 0.7 that is 0.42. It says how much variety a group holds, the way a census counts people.
Random mating keeps genotype counts predictable through p squared plus 2pq plus q squared equals 1. Estimate p and q from the data, form the three shares, and compare with what you counted. Agreement means the simple model roughly holds. This works only as a baseline: it assumes random mating with no selection, no mutation, no migration, and a large population.
Variation between populations is read off a structure index. Near zero means the groups mix freely and stay similar. A high value means they are isolated, so each drifts on its own and they diverge. Movement of individuals and genes between groups is called gene flow. Strong flow keeps groups alike, while its absence lets them part ways.
A sample from two mixed populations looks short of heterozygotes. Pooling groups with different versions means few individuals carry one version from each group, so heterozygotes look scarce. Whenever counts refuse to fit the simple model, look for hidden structure. Disagreement is not failure, since it points at selection, inbreeding, or a mix worth chasing.
Two p q measures variety inside a group, the structure index compares groups, and a heterozygote shortage betrays a hidden mix.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.