When Does the Hardy-Weinberg Law NOT Apply? Understanding the Limits of Genetic Equilibrium

The Hardy-Weinberg law is a foundational principle in population genetics, providing a mathematical model for predicting allele and genotype frequencies in a non-evolving population. However, this law is based on several critical assumptions. When any of these assumptions are violated, the population may deviate from Hardy-Weinberg equilibrium, and the law does not accurately describe genetic dynamics.

Key Assumptions of the Hardy-Weinberg Law

For the Hardy-Weinberg equilibrium to hold, a population must meet these conditions:

• Organisms are diploid

• Mating is random

• Population size is infinitely large

• No mutation occurs

• No migration (gene flow) occurs

• No natural selection acts on the population

• Allele frequencies are equal in the sexes (balanced sex ratio)

Evaluating the Options

Let’s consider each scenario:

• (1) 3 alleles are involved:
  The Hardy-Weinberg law can be extended to populations with more than two alleles at a locus. The equation simply expands to include all alleles (e.g., p2+q2+r2+2pq+2pr+2qr=1p2+q2+r2+2pq+2pr+2qr=1), so this does not violate the law.

• (2) Alleles are X-linked:
  The Hardy-Weinberg principle can be applied to X-linked alleles, but calculations must account for different numbers of X chromosomes in males and females. The law itself is still applicable with proper adjustments.

• (3) Skewed sex ratio:
  This violates a key assumption of the Hardy-Weinberg law. The law assumes equal allele frequencies in both sexes and a balanced sex ratio. A skewed sex ratio disrupts random mating and can lead to changes in allele frequencies across generations, meaning the population will not remain in Hardy-Weinberg equilibrium.

• (4) Population is tetraploid:
  The Hardy-Weinberg law can be mathematically extended to polyploid populations (such as tetraploids), with the equilibrium equations adjusted accordingly. The law is not limited to diploids only, though the classic form is for diploids.

Conclusion

The Hardy-Weinberg law will NOT operate under a skewed sex ratio, as this violates the assumption of equal allele frequencies between the sexes and random mating.

Correct answer: (3) Skewed sex ratio