A rabbit breeder is studying fur colour in rabbits. Brown fur colour is controlled by a dominant allele (F) and white fur colour is controlled by a recessive allele (f). Two rabbits that are both heterozygous for fur colour are crossed. Explain the expected ratio of phenotypes in the offspring of this cross.

Edexcel GCSE Biology (1BI0) — 3.4 Inheritance · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Both parents are Ff.

Punnett square:

| | F | f |
|---|---|---|
| F | FF | Ff |
| f | Ff | ff |

Genotype ratio: 1 FF : 2 Ff : 1 ff.

Phenotype: FF and Ff give brown fur (F is dominant); ff gives white fur.

Thus the expected phenotypic ratio is 3 brown : 1 white.

Examiner tips

  • State the parents’ genotypes first. Show the Punnett square or give the genotype ratio. Explain how dominance gives the phenotypic ratio. Use the exact ratio 3:1 to match the mark scheme.

Common mistakes

  • Mixing up dominant and recessive alleles. Failing to show the Punnett square or giving an incorrect ratio. Using the wrong phenotypic ratio such as 1:1.

Mark scheme (4 marks)

  1. Both parents have genotype Ff (heterozygous)
  2. Correct Punnett square outcomes: FF, Ff, Ff, ff (or equivalent stated genotype ratios 1 FF : 2 Ff : 1 ff)
  3. Three offspring (FF and Ff) will have brown fur because F is dominant / the dominant allele is expressed
  4. One offspring (ff) will have white fur, giving an expected ratio of 3 brown : 1 white

Key terms in this question

heterozygous · dominant · recessive · phenotype · allele

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