Explain how sex-linked inheritance can result in a phenotypic ratio that differs between males and females in the offspring of a carrier mother and an unaffected father, using a named X-linked recessive trait as an example.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Sex‑linked genes are on the X chromosome; males are hemizygous (X^A Y). A carrier mother is heterozygous (X^A X^a) and an unaffected father is X^A Y.
Female offspring receive one X from each parent: half are X^A X^A (unaffected) and half X^A X^a (carrier). No female is affected.
Male offspring receive the Y from the father and either X^A or X^a from the mother: half are X^A Y (unaffected) and half X^a Y (affected). Thus males show a 1:1 affected:unaffected ratio, giving a different phenotypic ratio between sexes.
Example: X‑linked haemophilia.
Female offspring receive one X from each parent: half are X^A X^A (unaffected) and half X^A X^a (carrier). No female is affected.
Male offspring receive the Y from the father and either X^A or X^a from the mother: half are X^A Y (unaffected) and half X^a Y (affected). Thus males show a 1:1 affected:unaffected ratio, giving a different phenotypic ratio between sexes.
Example: X‑linked haemophilia.
Examiner tips
- State the chromosomal location and hemizygosity of males; name a specific X‑linked recessive trait; give correct parental genotypes; explain the different ratios for each sex.
Common mistakes
- Confusing Y with X in the father’s genotype; claiming females can be affected; omitting the 1:1 male ratio; not naming a real X‑linked trait.
Mark scheme (4 marks)
- Sex-linked genes are located on the X chromosome (X-linked), and males are hemizygous — they carry only one allele for the trait.
- A carrier mother is heterozygous (X^A X^a) and an unaffected father is X^A Y; correct genotypes stated or implied for both parents using a named trait (e.g. colour blindness, haemophilia).
- Among female offspring, half are unaffected homozygous (X^A X^A) and half are unaffected carriers (X^A X^a) — no females are affected.
- Among male offspring, half are unaffected (X^A Y) and half are affected (X^a Y), giving a 1:1 ratio of affected to unaffected males — creating a different phenotypic ratio between the sexes.
Key terms in this question
sex-linked inheritance · carrier · X-linked recessive · phenotypic ratio
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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