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 that offspring will be red-eyed males is 25 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: X Upper R X r, X Upper R Y, X Upper R Y, X Upper R 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 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 r and x r. Clockwise, from top-left, the boxes include: Upper X r Upper X r, Upper X r Upper Y, X r Upper Y, Upper X r Upper X r.

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

Answer 1

The second Punnett square. It shows a heterozygous female (X^R X^r) crossed with a white male (X^r Y). Gametes: mother X^R or X^r, father X^r or Y. Offspring: X^R X^r (red female), X^r X^r (white female), X^R Y (red male), X^r Y (white male) — so 1 of 4 (25%) are red-eyed males.