Leaving Cert Biology Genetics Practice Test 2026 – All-in-One Exam Prep

Under Hardy-Weinberg conditions, the proportions of genotypes in a large population with random mating and no evolutionary forces are determined by the allele frequencies p and q (where p + q = 1). The chance of inheriting two copies of the dominant allele is p times p, giving p^2; the chance of one dominant and one recessive allele is 2pq (since either parent can contribute each allele); and the chance of two recessive alleles is q^2. So the expected genotype frequencies are p^2, 2pq, and q^2. This reflects how random mating combines allele alleles from the gene pool to produce genotype proportions that stay constant generation to generation when no evolution is acting.

If you plug in numbers, for example p = 0.6 and q = 0.4, the expected genotype frequencies would be 0.36 (AA), 0.48 (Aa), and 0.16 (aa).

The other statements don’t capture this genotype distribution. Random mating by itself doesn’t change allele frequencies, so saying allele frequencies change due to random mating isn’t accurate. And genotype frequencies aren’t equal for all genotypes—only under very special, non-general conditions would that happen. The precise expression describing genotype frequencies under no evolutionary forces is p^2, 2pq, q^2.