Explain how sex linkage causes a difference in the phenotypic expression of X-linked recessive traits between males and females.

IB DP Biology Higher Level (2023 syllabus) — D3.2 Inheritance · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

In humans, the gene encoding red–green colour vision is located on the X chromosome. Red–green colour blindness is caused by a recessive allele of this gene.

Model answer (4 marks)

Males are hemizygous for X‑linked genes, possessing only one X chromosome (and one Y chromosome), so they have only one copy of the allele. A single recessive allele on the X chromosome is sufficient to express the trait in males, because there is no second allele to mask it. Females have two X chromosomes and therefore two alleles at the locus; a female must be homozygous recessive to express the trait. Heterozygous females are carriers – they do not express the phenotype but can pass the recessive allele to offspring, explaining why the trait is more common in males than females in the population.

Examiner tips

  • Use the term ‘hemizygous’ for males; this shows understanding of sex linkage. Show that a single recessive allele is enough in males, but females need two. Mention carriers to explain prevalence in the population. Keep the answer concise – 4 marks only.
  • common_mistakes
  • :
  • Saying males need two copies of the allele, or that females only need one. Forgetting to mention carriers or the population frequency difference.

Mark scheme (4 marks)

  1. Males are hemizygous for X-linked genes, possessing only one X chromosome (and one Y chromosome), so they have only one copy of the allele.
  2. A single recessive allele on the X chromosome is sufficient to express the trait in males, because there is no second allele to mask it.
  3. Females have two X chromosomes and therefore two alleles at the locus; a female must be homozygous recessive to express the trait.
  4. Heterozygous females are carriers — they do not express the phenotype but can pass the recessive allele to offspring, explaining why the trait is more common in males than females in the population.

Key terms in this question

sex linkage · X-linked recessive

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