Sep 1, 2015
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# 31 from ADA 2007 sample test.
In human beings, color blindness is controlled by an X-linked recessive allele. In a cross involving this X-linked trait, the male parent has normal color vision, but the female parent is a carrier. What are the chances (in %) that a male offspring will inherit color blindness?

I thought the answer was 50%. b/c female parent is a carrier.. doesn't that mean that the female has 50% of becoming a carrier. The problem didn't specify and say the female is a carrier, and I thought that female has 50% of becoming a carrier, and female (either X^cX or X X ) with male X Y would produce another 50% male inheriting color blindness;thus, 25%?

BTW the answer was 50%... but I thought it was 25%
 

enamel88

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I agree it should be 25% bc they're saying gender AND blind which is the multiplicative rule of (1/2)(1/2). half will be male x half will have the disease. Maybe it's an error?
 
Jan 9, 2015
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XY x XX^b gives XY and X^bY, so out of the two male, one of them will have color blindness or 50%.
 
Aug 5, 2015
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I do believe the correct answer is 50%. In the question, "what are the chances (in %) that a MALE offspring will inherit colour blindness?", it already states that the offspring will be a male. The question would be quite different if it had asked, "what are the chances that a offspring will be MALE and inherit COLOUR BLINDNESS"?

Since the probability of the offspring being male is 100% (it is simply given to you), you do not have to worry about it. Since the female is a carrier for colour blindness, she will either pass the dominant allele for regular colour vision or the recessive allele for colour blindness to any of her offspring that is MALE. The father does not get much of a say since his contribution to the male offspring is the Y chromosome.

To confirm that the answer is 50%, draw out the punnett square.
 

snowpea28

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Mar 9, 2014
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I do believe the correct answer is 50%. In the question, "what are the chances (in %) that a MALE offspring will inherit colour blindness?", it already states that the offspring will be a male. The question would be quite different if it had asked, "what are the chances that a offspring will be MALE and inherit COLOUR BLINDNESS"?

Since the probability of the offspring being male is 100% (it is simply given to you), you do not have to worry about it. Since the female is a carrier for colour blindness, she will either pass the dominant allele for regular colour vision or the recessive allele for colour blindness to any of her offspring that is MALE. The father does not get much of a say since his contribution to the male offspring is the Y chromosome.

To confirm that the answer is 50%, draw out the punnett square.
Yup- the answer is 50%.

Another way to think about it:

You have 4 pens. 2 pens are red. 2 pens a blue. Only the BLUE pen has a cap.

Out of the BLUE pens, how many have caps?
So ONE blue pen has a cap out of TWO so that would be 50%.

HOWEVER, if it asks you how many pens overall have caps that is ONE pen in FOUR. So 25%.
 
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May 11, 2015
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50% males are normal and 50% of males are positive
50%females are normal, 50% carrier.
25% of all children are colorblind.
 

DC206R

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I had this issue before but then I realized that it is all in the wording of the question like the above posts have mentioned.