Question

the results of thomas hunt morgans last cross are shown in the punnett square below. which of thomas hunt morgans hypotheses was valid based on these results? white eyes are lethal in female drosophila. white eyes are lethal in male drosophila. female drosophila with white eyes are homozygous recessive.

Explanation:

In the Punnett square, there are no female flies with two recessive alleles (\(X^{r}X^{r}\)). But female flies (\(X^{R}X^{r}\)) are present. For male flies, \(X^{r}Y\) (white - eyed males) are present. Morgan's work showed that for a female to have white eyes (a recessive trait in this context), she needs to be homozygous recessive (\(X^{r}X^{r}\)). In this cross, since there are no \(X^{r}X^{r}\) females (but \(X^{R}X^{r}\) females are there), it supports the idea that female Drosophila with white eyes are homozygous recessive. The first option (white eyes are lethal in female Drosophila) is not supported as there are female genotypes (\(X^{R}X^{r}\)) in the cross. The second option (white eyes are lethal in male Drosophila) is also not supported as \(X^{r}Y\) (white - eyed males) are present in the Punnett square.

Answer:

Female Drosophila with white eyes are homozygous recessive.