Huntington's disease is a genetic disorder caused by a dominant gene mutation. Explain why a child can develop Huntington's disease even if only one parent carries the mutant allele, and describe how this pattern of inheritance differs from that of a recessive genetic disorder such as cystic fibrosis.

Pearson Edexcel A-Level Biology A (9BI0) — 1.5 Gene mutations and genetic disorders · Explain · 5 marks · View as Markdown

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

Huntington's disease is a late-onset neurological condition affecting muscle control and cognition. It is caused by a dominant allele on an autosome. Cystic fibrosis is caused by a recessive allele on an autosome.

Model answer (5 marks)

A dominant allele means only one copy is required for the phenotype to appear. If one parent is heterozygous (M/m) for the Huntington’s allele, each gamete has a 50 % chance of carrying the mutant allele, so a child has a 1 in 2 chance of inheriting it and developing the disease.

In contrast, cystic fibrosis is recessive. A child must inherit two mutant alleles (m/m) to show symptoms. A single mutant allele (m) results in a carrier (m/M) who is phenotypically normal. Thus two carrier parents can produce an affected child with a 25 % (1 in 4) probability.

Therefore, Huntington’s disease can arise from a single affected parent, whereas cystic fibrosis requires both parents to be carriers for an affected child to be possible.

Examiner tips

  • Mention the 50% transmission for a dominant allele; state the 25% risk for two carrier parents; use terms ‘heterozygous’, ‘carrier’, ‘recessive’ and ‘dominant’ exactly as in the mark scheme.
  • Show the difference in required allele copies clearly: one for dominant, two for recessive.

Common mistakes

  • Confusing the probability of transmission (using 1/4 instead of 1/2 for Huntington’s); Failing to explain that a single mutant allele is enough for the dominant disorder; Mixing up carrier status with affected status for cystic fibrosis.

Mark scheme (5 marks)

  1. The Huntington's disease allele is dominant, so only one copy of the mutant allele is needed for the disorder to be expressed.
  2. If one parent is heterozygous for the dominant allele, there is a 50% (1 in 2) chance of passing the mutant allele to each child.
  3. In cystic fibrosis, the allele is recessive, so a child must inherit two copies of the mutant allele (one from each parent) to develop the disorder.
  4. A person who inherits only one copy of the recessive cystic fibrosis allele is a carrier and does not show symptoms of the disorder.
  5. Therefore, unlike Huntington's disease, cystic fibrosis can be passed on by two unaffected carrier parents, and the probability of an affected child is 25% (1 in 4) when both parents are carriers.

Key terms in this question

gene mutation · genetic disorder

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