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A couple, each phenotypically normal, have a child with phenylketonuria, an autosomal recessive trait - VCE - SSCE Biology - Question 6 - 2012 - Paper 1
Question 6
A couple, each phenotypically normal, have a child with phenylketonuria, an autosomal recessive trait. The chance that their second child will have the trait is: A.... show full transcript
Worked Solution & Example Answer:A couple, each phenotypically normal, have a child with phenylketonuria, an autosomal recessive trait - VCE - SSCE Biology - Question 6 - 2012 - Paper 1
Step 1
Determine the genetic background
Answer
Phenylketonuria (PKU) is an autosomal recessive condition. This means that both parents must be carriers of the recessive allele (represented as 'a') for a child to express the trait (aa). If both parents are phenotypically normal (AA or Aa), they can either be homozygous dominant (AA) or heterozygous (Aa).
Step 2
Step 3
Calculate the probability for the second child
Answer
The offspring distribution shows that there is one in four (1/4 chance) for a child to be homozygous recessive (aa), which indicates that the child will have phenylketonuria. The question specifically asks for the chance that their second child will have the trait, which remains independent of the first child's condition.
Step 4