Red-green color blindness is due to an X-linked recessive allele in humans. A widowâs peak (a hairline that comes to a peak in the middle of the forehead) is due to an autosomal dominant allele. Consider the following family history:

A man with a widowâs peak and normal color vision marries a color-blind woman with a straight hairline.
The manâs father had a straight hairline, as did both of the womanâs parents.

Use the family history to make predictions about the coupleâs children.

What is the chance that any daughter the couple has will be color blind with a straight hairline?

A. 0B. 1/2C. 3/4D. 1E. 1/4

A. 0

Explanation:

Both red-green color blindness and and shape of hairline are independently assorting genes.

Red-green color blindness is X-linked recessive while hairline shape is autosomal dominant.

The man who has widow's peak had a father with straight hairline. This means that the man is heterozygous for the trait.

Assuming hairline shape is represented by S. A heterozygous widow's peak man will have a genotype of Ss. A straight hairline woman will have a genotype of ss.

Assuming the allele for colour blindness is c. A normal vision man will have the genotype while a color blind woman will have the genotype .

  x    

The man does not have the color blind allele and a daughter will need two color blind alleles (one from the father and the other from the mother) in order to show the trait.

This means that the probability of the couple producing a color blind daughter is 0. Hence, the chance that any daughter the couple has will be color blind with a straight hairline is also 0.

The correct option is A.

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