In the following human pedigree, squares represent males, circles represent females, and shaded symbols indicate individuals affected with a disorder.

One of the affected males from the third generation has a child with a female who is a carrier. For the pedigree shown above, which of the following best expresses the probability that the couple’s first son will be affected with the disorder?

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The disease is likely caused by a recessive allele. There is also a strong likelihood that the trait is sex-linked, since only males are affected.

The cross between an affected male and female carrier can be represented as:

$$\underbrace{X^A X^a}_{\text{carrier mother}} \times \underbrace{X^A Y}_{\text{affected father}} \tag{recessive, sex-linked}$$ $$\begin{array} {|c|c|c|} \hline & X^A & X^a \\ \hline X^A & X^AX^A &X^AX^a \\ \hline Y & X^AY& \colorbox{yellow}{$X^aY$} \\ \hline \end{array}$$

The probability that the son is affected is 50%.

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In this problem, an autosomal trait would result in the same probability - 50% chance.

$$\underbrace{Aa}_{\text{carrier mother}} \times \underbrace{aa}_{\text{affected father}} \tag{recessive, autosomal}$$ $$\begin{array} {|c|c|c|} \hline & A & a \\ \hline a & Aa & \colorbox{yellow}{$aa$} \\ \hline a & Aa & \colorbox{yellow}{$aa$} \\ \hline \end{array}$$