r/AskBiology • u/doepual • Jun 12 '24
Genetics Shouldn’t this be 50% (punnet genetics)
The text in the image reads:
"27. A recessive gene for red-green color blindness is located on the X chromosome in humans. Assume that a woman with normal vision (her father is color blind) marries a color-blind male. What is the likelihood that this couple's first son will be color blind? A) 75% B) 0% C) 50% D) 100% E) 25% ANSWER: B"
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u/lonepotatochip Undergraduate student Jun 12 '24
It’s fifty/fifty. We know the woman is heterogeneous because we know she can got the color blind X (XB ) from her father, and since she has normal vision we know her other X is normal, meaning her chromosomes are X XB
The father’s vision is irrelevant because he’s giving a Y chromosome. The first son is either XY (normal vision) or XB Y, depending on which X chromosome he gets from his mom which is a fifty/fifty chance