H1 Biology 8876 · Nov 2023BiologyPaper 1Most challenging

Paper 1 Q19 — X-linked genes in males

Genetics — males are hemizygous for X-linked genes (one allele only)

What the examiners said

“Option A… was selected by the majority… These candidates had not considered that for genes carried on the X chromosome, individual males (of mammalian species such as humans) will only have one allele of each gene.”

Try the question

For a gene located on the X chromosome, how many alleles does a human male carry, and what follows?

Examiner report: “Candidates treated X-linked genes as if males had two alleles rather than one (hemizygous).”

Explore — trace an X-linked allele

Set the parents’ alleles and see the offspring. Because males have a single X, one recessive allele shows up in them — unlike females, who need two.

Mother (X X): carrier X\u1d34X\u02b0 Father (X Y): unaffected X\u1d34Y

A son’s single X means one recessive allele is enough to affect him. Forgetting males are hemizygous throws off every X-linked cross ratio.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H1 Biology 8876 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

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