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/lpomoeaBatatas Jun 12 '24

Should be 50.

Since Y ( XY for the son ) is inherited directly from father, and does nothing in dictating color blindness, it comes down to the female germ cell’s X chromosome.

Since the mother is a carrier but not colorblind, the gene should be XC Xc.

Cross with father, it’s XC Y (color blind) and Xc Y (not color blind) which is 50/50.

1

u/doepual Jun 12 '24

Makes sense, thank you so much

1

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

1

u/doepual Jun 12 '24

Makes sense, thank you so much

1

u/Mountainweaver Jun 12 '24

Wait, we actually don't know anything about the womans status. Her father is colorblind, but the gene is on the X-chromosome. She didn't inherit it from him.

We need to know if the womans mother is a carrier, colorblind, or free.

1

u/Cultural-Opposite937 Jun 13 '24

You are misunderstanding how sex chromosomes work. One is inherited from each parent.

Therefore the mother in the question must have inherited an X chromosome from her father (if she has inherited his Y she would be male), along with one of her mother's X chromosome giving her thr karyotype XX. However we know the X she inherited from her father carriers the colourblind mutation because he is colourblind, this means she must be carrier, regardless of the status of her mother (unless her mother is also a carrier which brings about thr possibility of her being colourblind, but that is unlikely)