Question

Drosophila eye color is an X-linked trait. Red eye color is dominant, and white eye color is recessive. Which Punnett square shows a cross in which the probability of offspring will have red eyes is 100 percent?

A Punnett square is shown. The columns are labeled Upper X r and Upper Y. The rows are labeled x Upper R and x Upper R. Clockwise, from top-left, the boxes include: Upper X Upper R Upper X r, Upper X Upper R Y, X Upper R Upper Y, X Upper R Upper X r.

A Punnett square is shown. The columns are labeled Upper X Upper R and Upper Y. The rows are labeled x Upper R and x r. Clockwise, from top-left, the boxes include: Upper X Upper R Upper X Upper R, Upper X Upper R Upper Y, Upper X r Upper Y, Upper X uppercase Upper R Upper X r.

A Punnett square is shown. The columns are labeled Upper X r and Upper Y. The rows are labeled x r and x r. Clockwise, from top-left, the boxes include: Upper X r Upper X r, Upper X r Upper Y, Upper X r Upper Y, Upper X r Upper X r.

A Punnett square is shown. The columns are labeled Upper X r and Upper Y. The rows are labeled x Upper R and x r. Clockwise, from top-left, the boxes include: Upper X Upper R Upper X r, Upper X Upper R Y, Upper X r Upper Y, Upper X r Upper X r.

Answer 1

The first Punnett square. It shows a homozygous dominant female (X^R X^R) crossed with a white-eyed male (X^r Y). The mother can only contribute X^R, so all offspring (X^R X^r daughters and X^R Y sons) carry at least one R allele and have red eyes (100%).