Question

which of the following is true of the genetic cross shown in the figure? their daughters have a 50 percent chance of having the disorder. their sons have a 50 percent chance of being carriers. all of their daughters will be carriers. their sons have a 50 percent chance of having the disorder.

Explanation:

Step1: Analyze the cross

The female has genotype \(X^{R}X^{R}\) (not affected) and the male has genotype \(X^{r}Y\) (affected). The possible gametes from the female are \(X^{R}\) (all gametes, since both X - chromosomes are \(X^{R}\)). The possible gametes from the male are \(X^{r}\) and \(Y\).

Step2: Determine offspring genotypes

When we do the cross \(X^{R}X^{R}\times X^{r}Y\), using the Punnett - square method (or simple combination of gametes):

• For daughters: Combine \(X^{R}\) (from mother) with \(X^{r}\) (from father) to get \(X^{R}X^{r}\).
• For sons: Combine \(X^{R}\) (from mother) with \(Y\) (from father) to get \(X^{R}Y\).

Step3: Evaluate each option

• Option 1: Daughters have genotype \(X^{R}X^{r}\). Since \(R\) is dominant (assuming \(R\) is the non - disorder allele), daughters do not have the disorder.
• Option 2: Sons have genotype \(X^{R}Y\). Males cannot be carriers for X - linked recessive disorders (they have only one X - chromosome).
• Option 3: Daughters have genotype \(X^{R}X^{r}\), so all daughters are carriers.
• Option 4: Sons have genotype \(X^{R}Y\), so sons do not have the disorder.

Answer:

All of their daughters will be carriers.