A couple are both heterozygous for the allele that causes cystic fibrosis. The allele for cystic fibrosis is recessive. Explain why there is a 75% probability that their child will not have cystic fibrosis.

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).

Cystic fibrosis is an inherited disorder caused by a recessive allele. Use the letter F to represent the dominant allele and f to represent the recessive allele.

Model answer (4 marks)

Both parents are heterozygous, so their genotype is Ff.

When two Ff parents mate, the Punnett square gives four possible genotypes:
FF, Ff, Ff, ff.

Cystic fibrosis occurs only in the homozygous recessive genotype ff.

Three of the four genotypes (FF, Ff, Ff) contain at least one dominant allele, so 3 out of 4, or 75 %, of the children will not have cystic fibrosis.

Examiner tips

  • State the parents’ genotype first. Show the four possible offspring genotypes. Explain that only ff causes the disease. Count the favourable genotypes to give the 75 % probability.

Common mistakes

  • Mixing up dominant and recessive alleles. Failing to list all four genotypes. Assuming all heterozygotes have the disease.

Mark scheme (4 marks)

  1. Both parents have the genotype Ff (heterozygous)
  2. Correct identification of all four possible offspring genotypes from the cross: FF, Ff, Ff, ff
  3. Cystic fibrosis only occurs in offspring with the genotype ff (homozygous recessive)
  4. Three out of four possible genotypes contain at least one dominant allele (FF or Ff), so 75% of offspring will not have cystic fibrosis

Key terms in this question

heterozygous · recessive · allele

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