Two Healthy Parents and a Child with Sickle Cell Anaemia
At a district hospital, a mother brings her 3-year-old son with severe anaemia, jaundice and episodes of pain in the hands and legs. A blood test shows sickle cell anaemia (HbSS). Haemoglobin electrophoresis shows that both parents are AS. The mother and father are healthy and have never had a crisis. The mother's sister has sickle cell trait, and the father's brother has sickle cell anaemia. A nurse tells the parents that the child inherited the condition only from the mother, so the father's family is not involved. The family lives in a malaria-endemic area and keeps no mosquito net. The parents ask whether future children can be affected and what they should do about malaria.
1. Using a genetic diagram, explain how two AS parents can have an HbSS child and state the probability for the next child.
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Let A be the normal haemoglobin allele and S the sickle allele. Each parent is AS, so each can produce A or S gametes. A Punnett square gives: AA (25%), AS (50%), SS (25%).
The child with HbSS has inherited an S allele from the mother and an S allele from the father. The probability that the next child is HbSS is 1 in 4, or 25%, and this probability applies independently to each pregnancy. The chance of a child who is a carrier (AS) is 50%, and the chance of a child with neither sickle allele (AA) is 25%. Having one affected child does not change the odds for the next child, although across several children the observed numbers may vary by chance.
2. Evaluate the nurse's statement that only the mother passed on the condition.
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The statement is incorrect. Sickle cell anaemia is an autosomal recessive condition, not sex-linked and not inherited from one parent alone. The affected child has the genotype SS, so one S allele came from each parent. Both parents are AS carriers.
The father's family history is relevant because the father carries the S allele and can pass it to any child. The mother's sister and the father's brother show that the allele is present on both sides of the family. The nurse may have confused the fact that the mother carried the child with the genetic contribution. Genetic counselling should include both parents and should explain that carrier parents are healthy but can have affected children.
3. Explain why the child with HbSS is at greater risk from malaria and give two precautions.
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The child has sickle cell anaemia, not just the trait. In HbSS, the spleen often becomes damaged by repeated sickling and may stop functioning properly (functional asplenia). The spleen normally filters infected red blood cells and helps fight encapsulated bacteria, so the child is less able to clear malaria parasites and is at risk of severe malaria, severe anaemia and bacterial infection. Malaria infection can also trigger sickling and painful crises.
The AS trait gives some protection against severe falciparum malaria, but HbSS does not; the child already has two S alleles. Precautions include sleeping under an insecticide-treated mosquito net every night, removing stagnant water near the home, and going to the clinic immediately for any fever so malaria is tested and treated promptly. The child should also have routine vaccinations and, where advised by clinicians, preventive treatment.