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# Phenylketonuria (PKU) is a recessive genetic disorder. If both parents are carriers, what is the probability of this couple having a child with PKU? What is the chance of this couple having two children with PKU?

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Since phenyketonuria (PKU) is a recessive genetic disorder, the trait is inherited when the child receives two recessive alleles from the parents.

Given that the parents are carrier (heterozygous dominant), the probability of having a child that develops PKU is 25 $\%$ or $\frac{1}{4}$. (Recall that a cross of Aa $\times$ Aa gives 1AA: 2Aa: 1aa).

Note that the chance of having a second child with PKU is still 25 $\%$ or $\frac{1}{4}$. Now, the the chance of this couple having two children with PKU is $\frac{1}{4} \times \frac{1}{4}= \frac{1}{16}$

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