Question

x-linked cross: parental genotypes and f1 phenotypes
in drosophila, white eyes is caused by a recessive allele (r) located on the x chromosome. the dominant allele (r) produces red eyes. suppose a white - eyed female mates with a red - eyed male. knowing this, complete the sentences with the correct terms.
the genotype of the male is best represented as __________
the genotype of the female is best represented as __________
among the male f₁ offspring, the predicted proportion with white eyes is __________
among the female f₁ offspring, the predicted proportion with white eyes is __________
among all f₁ offspring, the predicted proportion with white eyes is __________

Explanation:

Step1: Determine male genotype

Males have \(XY\) chromosomes. Since the male has red eyes (dominant \(R\)), and the allele is on the \(X\) - chromosome, the genotype of the male is \(X^{R}Y\) (or \(RY\) in the given format).

Step2: Determine female genotype

Females have \(XX\) chromosomes. The female has white eyes (recessive \(r\)), so she must have two recessive alleles on her \(X\) - chromosomes. The genotype of the female is \(X^{r}X^{r}\) (or \(rr\) in the given format).

Step3: Analyze male \(F_1\) offspring

The cross is \(X^{r}X^{r}\times X^{R}Y\). The male offspring get their \(X\) - chromosome from the mother (\(X^{r}\)) and \(Y\) from the father. So all male \(F_1\) offspring have the genotype \(X^{r}Y\) (white - eyed). The proportion of white - eyed male \(F_1\) offspring is \(100\%\).

Step4: Analyze female \(F_1\) offspring

Female offspring get one \(X\) (\(X^{r}\)) from the mother and one \(X\) (\(X^{R}\)) from the father. The genotype of female \(F_1\) offspring is \(X^{R}X^{r}\). Since \(R\) is dominant, the proportion of white - eyed female \(F_1\) offspring is \(0\%\).

Step5: Analyze all \(F_1\) offspring

Let's assume equal numbers of male and female offspring (a \(1:1\) ratio in \(X\) - linked crosses without other factors). If males (\(100\%\) white - eyed) and females (\(0\%\) white - eyed) are in a \(1:1\) ratio, the proportion of white - eyed offspring among all \(F_1\) is \(\frac{1}{2}\times100\%+\frac{1}{2}\times0\% = 50\%\).

Answer:

The genotype of the male is best represented as \(RY\).
The genotype of the female is best represented as \(rr\).
Among the male \(F_1\) offspring, the predicted proportion with white eyes is \(100\%\).
Among the female \(F_1\) offspring, the predicted proportion with white eyes is \(0\%\).
Among all \(F_1\) offspring, the predicted proportion with white eyes is \(50\%\).