A hospital laboratory tests a newborn whose red blood cells break down rapidly. Fig. 1 shows simplified chromosomes from the baby. One chromosome pair carri...
Question 1 Report
A hospital laboratory tests a newborn whose red blood cells break down rapidly. Fig. 1 shows simplified chromosomes from the baby. One chromosome pair carries an allele for a blood disorder. The parents are concerned because neither parent currently shows symptoms.
(a) Name the structures shown in Fig. 1. [1]
(b) Use the letters in Fig. 1 to state the baby's genotype. [1]
(c) Suggest why two parents without symptoms could have a baby with a recessive genetic disorder. [3]
Answer Details
(a) The structures shown are chromosomes. [1]
(b) The genotype shown in Fig. 1 is \(Ss\). [1]
(c) Two parents without symptoms could have a baby with a recessive disorder if both are heterozygous carriers. Each carries one recessive disorder allele without showing symptoms. An affected baby would need to inherit a recessive allele from each parent and would have genotype \(ss\). [3]
Important check: the figure labels the baby's chromosomes \(S\) and \(s\), so it shows \(Ss\), not an affected \(ss\) genotype. The final point explains how an affected baby could arise in the general case, as required by the mark scheme.
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