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27
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N/A — Exam

Question 27 • Biology

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?

Answer
A
Image with description 2 by 2 Punnett square. Columns labeled X superscript lowercase r and Y. Rows labeled X superscript uppercase R and X superscript uppercase R. Top row: X superscript uppercase R X superscript lowercase r, X superscript uppercase R Y. Bottom row: X superscript uppercase R X superscript lowercase r, X superscript uppercase R Y.
B
Image with description 2 by 2 Punnett square. Columns labeled X superscript lowercase r and Y. Rows labeled X superscript uppercase R and X superscript lowercase r. Top row: X superscript uppercase R X superscript lowercase r, X superscript uppercase R Y. Bottom row: X superscript lowercase r X superscript lowercase r, X superscript lowercase r Y.
C
Image with description 2 by 2 Punnett square. Columns labeled X superscript lowercase r and Y. Rows labeled X superscript lowercase r and X superscript lowercase r. Top row: X superscript lowercase r X superscript lowercase r, X superscript lowercase r Y. Bottom row: X superscript lowercase r X superscript lowercase r, X superscript lowercase r Y.
D
Image with description 2 by 2 Punnett square. Columns labeled X superscript uppercase R and Y. Rows labeled X superscript lowercase r and X superscript lowercase r. Top row: X superscript lowercase r X superscript uppercase R, X superscript lowercase r Y. Bottom row: X superscript lowercase r X superscript uppercase R, X superscript lowercase r Y.

Explanation

A red-eyed male must have genotype XᴿY: he gets the Y chromosome from one parent and the dominant red-eye allele on his X chromosome from the other. In choice B, one of the four offspring boxes is XᴿY, so the probability is 1 out of 4, or 25 percent. The other male offspring is XʳY and would have white eyes.

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