A recessive allele on the X chromosome is responsible for red-green color blindness in humans. A...

A recessive allele on the X chromosome is responsible for red-green color blindness in humans. A woman with normal vision whose father is red-green color blind marries a male with normal vision.

Which of the following best predicts the phenotypes of their children?

	A.	All of the children will have normal vision.
	B.	All of the daughters will have normal vision, and half of the sons will be red-green color blind.
	C.	All of the daughters will have normal vision, and all of the sons will be red-green colorblind.
	D.	Half of the children will be red-green colorblind, regardless of their sex.

Solutions

Expert Solution

The gene for red-greencolorblindness is on X chromosome. Consider the dominant allele to be XC and the recessive allele to be Xc. The father of the woman is colorblind. In males, one copy of mutant X is sufficient to cause disease phenotype. In females, two copies of the mutant X are required for the color blindness phenotype. Hence, females with one Xc are carrier and will have normal vision.

The woman is with normal vision, while her father is color blind (XcY). Hence, she should be a carrier with genotype XCXc. The male has normal vision and hence, his genotype is XCY. Punnett square is:

		Woman XCXc
		XC	Xc
Male XCY	XC	XC XC Female Homozygous dominant Normal vision	XC Xc *Female heterozygous* *Carrier* *Normal vision*
	Y	XCY Male with dominant allele Normal vision	XcY Male with color blindness

All the daughters have normal vision, as they will be either homozygous dominant or carriers. Half of the males will have normal vision as they inherit the normal copy of X chromosome from their mother. The rest half of the male offspring will be colorblind due to inheritance of the mutant X from the mother.

Color-blindness is X linked inheritance and hence expression of the trait is based on the sex of the offspring. As the woman had one mutant X chromosome, 50% of her children’s will inherit this mutant X, although expression is based on sex of the child. Not all male offspring will have the color blind trait because the father is not affected.

Right choice: B. All of the daughters will have normal vision, and half of the sons will be red-green color blind.

gladiator answered 1 year ago

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