In humans, the disease phenylketonuria is inherited through a recessive gene. A phenotypically normal couple produced one normal child and one phenylketonuric child (i) Using symbol A for normal gene and a for phenylketonuric gene, write down the genotypes of the parents (ii) Work out the possible genotype of the normal child
Question
In humans, the disease phenylketonuria is inherited through a recessive gene. A phenotypically normal couple produced one normal child and one phenylketonuric child (i) Using symbol A for normal gene and a for phenylketonuric gene, write down the genotypes of the parents (ii) Work out the possible genotype of the normal child
Solution
(i) Since phenylketonuria is a recessive disease, it only manifests when the individual has two copies of the recessive gene (aa). The fact that the couple is phenotypically normal but produced a child with phenylketonuria means that both parents must be carriers of the disease. Therefore, they each have one normal gene and one phenylketonuria gene. So, the genotypes of the parents are Aa and Aa.
(ii) The possible genotypes of the normal child could be AA or Aa. This is because the child could inherit the normal gene from both parents (AA) or the normal gene from one parent and the phenylketonuria gene from the other parent (Aa). Since the child is phenotypically normal, they cannot have the aa genotype because that would result in phenylketonuria.
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