Sickle cell disease is a genetic disorder caused by a recessive gene mutation. A person who is heterozygous for the sickle cell allele is described as a carrier. Two carriers have children together. Explain why some of their children may have sickle cell disease, some may be carriers, and some may be unaffected and not carriers.

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).

Sickle cell disease is caused by a mutation in the gene that codes for haemoglobin. The mutation results in red blood cells that are an abnormal sickle shape, which can block blood vessels and reduce oxygen transport. The allele for sickle cell disease (s) is recessive to the normal allele (S).

Model answer (5 marks)

Both parents are carriers, so each has one normal allele (S) and one sickle cell allele (s) – they are heterozygous (Ss). During fertilisation each parent passes one of their two alleles to the zygote. The possible combinations are SS, Ss and ss. A child who receives two recessive alleles (ss) will have sickle cell disease. A child who receives one normal and one sickle cell allele (Ss) will be a carrier but will not show symptoms. A child who receives two normal alleles (SS) will be unaffected and not a carrier.

Examiner tips

  • State the parents’ genotypes first (Ss).
  • Explain the inheritance of one allele from each parent.
  • Describe the three possible genotypes and link each to the phenotype.

Common mistakes

  • Confusing dominant and recessive roles – writing SS as sickle cell.
  • Failing to mention that carriers are unaffected.

Mark scheme (5 marks)

  1. Both parents are carriers, so each parent has one dominant normal allele (S) and one recessive sickle cell allele (s) / both parents are heterozygous (Ss)
  2. Each parent passes one allele to the child / alleles are inherited from each parent during reproduction
  3. A child who inherits two recessive alleles (ss) will have sickle cell disease
  4. A child who inherits one dominant and one recessive allele (Ss) will be a carrier but unaffected
  5. A child who inherits two dominant alleles (SS) will be unaffected and not a carrier / the normal allele is dominant so SS children are completely unaffected

Key terms in this question

recessive · carrier · heterozygous · allele

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