A breed of cattle can display one of two coat colours: red or white. When a red bull is crossed with a white cow, all offspring have a roan coat, which appears as a mixture of red and white hairs. Explain why the roan offspring, when crossed with each other, produce red, roan, and white calves in an expected ratio of 1:2:1.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In this breed of cattle, coat colour is controlled by a single gene with two alleles. Neither allele is dominant over the other.
Model answer (4 marks)
Codominance is the inheritance pattern.
The roan cattle are heterozygous (Rr), carrying one red allele and one white allele.
When two heterozygous roan parents are crossed, the Punnett square gives the genotypes RR, Rr, rR and rr.
RR gives a red calf, Rr/rR gives a roan calf (both alleles expressed) and rr gives a white calf.
Thus the expected phenotypic ratio is 1 red : 2 roan : 1 white.
The roan cattle are heterozygous (Rr), carrying one red allele and one white allele.
When two heterozygous roan parents are crossed, the Punnett square gives the genotypes RR, Rr, rR and rr.
RR gives a red calf, Rr/rR gives a roan calf (both alleles expressed) and rr gives a white calf.
Thus the expected phenotypic ratio is 1 red : 2 roan : 1 white.
Examiner tips
- State codominance first. Mention roan = heterozygous. Show the Punnett square or list the four genotypes. Link each genotype to the correct phenotype to justify the 1:2:1 ratio.
Common mistakes
- Confusing codominance with incomplete dominance. Treating roan as homozygous. Failing to explain the 1:2:1 ratio by genotype–phenotype link.
Mark scheme (4 marks)
- Identifies the inheritance pattern as codominance
- States that roan individuals are heterozygous, carrying one allele for red and one for white
- States or implies that crossing two heterozygous roan parents produces all three possible genotype combinations
- Links genotype to phenotype to explain the 1:2:1 ratio — homozygous red is red, heterozygous is roan (both alleles expressed), homozygous white is white
Key terms in this question
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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