Paper 1 Q25 — Heterozygote advantage keeps all genotypes

Genetics — heterozygote advantage cannot eliminate homozygotes from a cross

What the examiners said

“These candidates recognised that heterozygous rats would always have a selective advantage but had not considered that… crosses between rats would necessarily result in offspring with all three possible genotypes.”

Try the question (reconstruction)

In a population, heterozygotes (Aa) have the highest fitness (heterozygote advantage). If two heterozygous survivors (Aa × Aa) breed, what genotypes appear in their offspring?

Examiner report: “Candidates thought selection for heterozygotes would prevent homozygous offspring.”

Explore — the Punnett square never lies

Cross two heterozygotes and watch the offspring ratio. Then apply selection against homozygotes — the next generation of Aa×Aa crosses still regenerates all three genotypes.

This is balanced polymorphism: heterozygote advantage keeps both alleles in the population precisely because every Aa×Aa cross remakes the homozygotes.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H2 Biology 9744 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

Made by lookang, using Claude Fable 5. More resources: iwant2study.org