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.
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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.
More to read
19.1 Population Evolution
Evolution is best watched at the level of populations, not single animals. A population evolves when the frequencies of gene versions shift over generations. Selection, drift, migration, and mutation each push those frequencies in their own way. Measuring the shifts turns evolution from a story into numbers you can test.
19.2 Population Genetics
Population genetics counts alleles and genotypes the way a census counts people. From those counts come summaries like expected heterozygosity, which says how much variation a group holds. When observed counts depart from expectation, something interesting is happening. A shortage of heterozygotes, for example, can betray a sample that unknowingly pooled two separate groups.
11.2 Mechanisms of Evolution
Allele frequencies change through a short list of mechanisms, and each leaves a signature. Selection favours helpful versions, drift lets chance rule in small groups, and migration blends groups together. Isolation does the opposite, letting groups drift apart until indices of structure climb. Learning the signatures lets you read history out of present day variation.
Learn: Mechanisms of evolution
This review pulls together how evolution works, from mutation supplying new versions to selection and drift sorting them. It stresses that mating patterns shape what we see: random mating keeps genotype counts predictable, while structure breaks the prediction. That contrast is the key to spotting hidden population divisions. Whenever numbers refuse to fit the simple model, look for the missing structure.
How to Calculate the Hardy Weinberg Equation: 12 Steps
The Hardy Weinberg equation turns allele frequencies into expected genotype counts in a few steps. You estimate p and q from the data, square and multiply to get the three genotype shares, then compare with what you counted. Agreement means the simple model holds, at least roughly. Disagreement is not failure, since it points at selection, inbreeding, or hidden structure worth chasing.
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