Genetic Diversity and Allele Frequencies

Genetic diversity is the raw material of evolution. Without variation in alleles, natural selection has nothing to act on and populations cannot adapt to environmental change. This lesson examines how genetic diversity is measured, why it matters, and how human activities (both destructive and constructive) alter allele frequencies over time. OCR A-Level Biology A specification 4.2.1 (f) requires you to calculate genetic diversity within a population and to explain the factors that affect it.

Key Definitions:

• Gene — a sequence of DNA that codes for a protein (or functional RNA).
• Allele — a variant form of a gene.
• Locus — the position of a gene on a chromosome.
• Polymorphic locus — a locus where more than one allele exists in the population at a frequency above 1%.
• Genetic diversity — the variety of alleles and genotypes within a population.
• Gene pool — the complete set of alleles in a population.

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Measuring Genetic Diversity

One common measure is the proportion of polymorphic gene loci:

$\text{Proportion of polymorphic loci} = \frac{\text{Number of polymorphic loci}}{\text{Total number of loci}}$Proportion of polymorphic loci=Total number of lociNumber of polymorphic loci​

A locus is considered polymorphic if two or more alleles are each present at frequencies of at least 1% (i.e. a second allele is not just a rare mutation).

Worked example: A study of a population of snails examines 50 gene loci and finds that 18 of them have two or more alleles at frequencies ≥ 1%. The proportion of polymorphic loci is:

18 / 50 = 0.36 \text{ (or 36%)}

Other measures include:

• Mean number of alleles per locus — averaged across all loci studied.
• Heterozygosity — the proportion of individuals that are heterozygous at a given locus.
• Allele frequency — the proportion of one allele relative to all alleles at a locus.

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Allele Frequency Calculations

For a gene with two alleles (A and a), the allele frequencies are conventionally called p (frequency of A) and q (frequency of a), with p + q = 1.

Example: In a population of 100 butterflies, the genotypes are:

• AA: 36
• Aa: 48
• aa: 16

Each individual has two alleles, so the total number of alleles = 200.

Number of A alleles = (2 × 36) + 48 = 72 + 48 = 120 Number of a alleles = (2 × 16) + 48 = 32 + 48 = 80

$p = 120/200 = 0.60, \quad q = 80/200 = 0.40$p=120/200=0.60,q=80/200=0.40

Check: p + q = 1.00 ✓

Exam Tip: Allele frequency calculations reward careful bookkeeping. Count all alleles (each individual contributes two), then divide. Mistakes come from forgetting that heterozygotes contribute one of each allele.

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Why Genetic Diversity Matters

flowchart LR
    A[Genetic Diversity] --> B[More alleles]
    B --> C[More phenotypic variation]
    C --> D[Natural selection has more to choose from]
    D --> E[Population adapts to change]
    A --> F[Reduced inbreeding]
    F --> G[Fewer recessive diseases expressed]

• Adaptability. Populations with more alleles are more likely to contain individuals with resistance to new diseases, pollutants or climates.
• Long-term survival. Small, inbred populations accumulate deleterious recessive alleles — a phenomenon called inbreeding depression.
• Ecosystem function. Genetically diverse plant populations are more productive and resilient.
• Conservation value. Species with low genetic diversity (cheetah, Hawaiian honeycreeper) need special management.

Case Study: The Cheetah Bottleneck

Around 10,000 years ago, cheetah populations crashed — possibly to fewer than 100 individuals — during the end of the Pleistocene. Every modern cheetah is descended from those few survivors. As a result, cheetahs are so genetically similar that skin grafts between unrelated individuals are not rejected — a level of homogeneity normally seen only in inbred laboratory mice. Cheetahs show high juvenile mortality, low sperm count and susceptibility to disease, all linked to genetic poverty.

Case Study: Florida Panthers

By the 1990s, the Florida panther population had fallen to fewer than 30 individuals, with kinked tails, heart defects and low fertility — all signs of inbreeding depression. Conservationists introduced eight female Texas cougars in 1995 to add genetic variation. Within a decade, the population had tripled and health problems declined.

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Sources of Genetic Variation

Where does genetic diversity come from? From several sources: