Physiology & Biochemistry · single question
Both parents are carriers (heterozygous) for sickle cell anaemia. What is the probability that a given child will be AFFECTED by the disease?
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Correct answer: C. 25%
- A. 75% is the chance of inheriting at least one copy of the allele (carrier or affected combined).
- B. 100% affected would require both parents to be homozygous for the disease allele.
- C. Correct. Sickle cell anaemia is AUTOSOMAL RECESSIVE. With two carriers (Aa x Aa) the Punnett square yields 25% homozygous unaffected (AA), 50% carriers (Aa), and 25% homozygous AFFECTED (aa). Note the common trap: 50% is the probability of being a CARRIER, not of having the disease.
- D. 50% is the chance the child is a carrier (heterozygous), not affected.
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