Define Hardy-Weinberg principle in simple terms.

• A. Allele frequencies change with natural selection but genotype frequencies stay the same.
• B. In a large, randomly mating population with no evolutionary forces, allele and genotype frequencies stay constant; p^2:2pq:q^2.
• C. Genotype frequencies must follow Mendel's ratios every generation.
• D. Homozygous genotypes become more common over time.

Answer: (B)

Hardy-Weinberg describes a no-evolution scenario: in a very large population with random mating and no forces like natural selection, mutation, migration, or drift, both allele frequencies and genotype frequencies stay the same across generations. If the frequency of one allele is p and the other is q (with p + q = 1), the expected genotype frequencies are p^2 for the homozygous first allele, 2pq for the heterozygotes, and q^2 for the homozygous second allele. This gives a simple prediction: you can calculate future genetic makeup from the current allele frequencies using p^2:2pq:q^2. For example, if p = 0.7 and q = 0.3, the expected genotype frequencies are 0.49, 0.42, and 0.09, respectively. This baseline helps us see when evolution is occurring, because real populations will deviate from these proportions if evolutionary forces are at work.

If an answer says allele frequencies change due to natural selection or that Mendelian ratios always apply every generation, those descriptions don’t fit the Hardy-Weinberg scenario, which is precisely about equilibrium under no evolutionary forces. Similarly, saying homozygous genotypes become more common over time describes a shift that would indicate evolution, not equilibrium.