A farmer notices that some of her sheep have black wool and some have white wool. Wool colour is controlled by a single gene. The allele for white wool (W) is dominant over the allele for black wool (w). The farmer crosses a heterozygous white-wool sheep with a black-wool sheep. Explain the possible genotypes and phenotypes of the offspring from this cross, and predict the proportion of offspring expected to show each phenotype.

OCR A-Level Biology B — Advancing Biology (H422) — 5.2 Inheritance and variation · Explain · 5 marks · View as Markdown

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

Wool colour in sheep shows discontinuous variation. The white wool allele (W) is dominant and the black wool allele (w) is recessive. A heterozygous white-wool sheep is crossed with a black-wool sheep.

Model answer (5 marks)

The heterozygous white‑wool parent has genotype Ww. The black‑wool parent has genotype ww.

Cross:
Ww × ww

Possible offspring genotypes:
1. Ww – white wool (W is dominant, only one W needed)
2. ww – black wool

Thus the expected phenotypic ratio is 1 white : 1 black (50 % white, 50 % black).

Examiner tips

  • Write the exact genotypes Ww and ww; mention W is dominant for white wool.
  • Show the Punnett square or simple ratio to justify 1:1 proportion.
  • Use the word ‘predict’ or ‘expect’ to indicate the ratio.
  • Include the 50 % figure for clarity.

Common mistakes

  • Confusing the heterozygous parent as WW instead of Ww.
  • Failing to state that Ww offspring are white because W is dominant.
  • Giving a 3:1 ratio instead of 1:1 for a single‑gene cross with one homozygous recessive parent.

Mark scheme (5 marks)

  1. Correctly identifies the genotype of the heterozygous white-wool parent as Ww
  2. Correctly identifies the genotype of the black-wool parent as ww
  3. States the two possible offspring genotypes as Ww and ww
  4. States that Ww offspring will have a white phenotype because W is dominant / only one dominant allele is needed for white wool to be expressed
  5. Predicts a 1:1 ratio / 50% white-wool offspring (Ww) to 50% black-wool offspring (ww)

Key terms in this question

allele · dominant · genotype · phenotype · heterozygous

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