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A color-blind woman marries a man who is not color-blind. All of their sons, but none of their daughters, are color-blind. Which of the following statements correctly explains these results?

1) The color-blindness gene is passed down from the father to the son.
2) The color-blindness gene is passed down from the mother to the son.
3) The color-blindness gene is passed down from the father to the daughter.
4) The color-blindness gene is passed down from the mother to the daughter.

User Askar
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1 Answer

6 votes

Final answer:

A color-blind woman passes the colorblind gene to her sons because she has the colorblind allele on one of her X chromosomes. Sons inherit their X chromosome from their mother and Y from their father. Daughters will not be colorblind unless they inherit colorblind alleles from both parents. The correct option is 2.

Step-by-step explanation:

Color blindness is a sex-linked recessive trait, typically found on the X chromosome. Since males (XY) only have one X chromosome, if it carries the gene for colorblindness, they will be colorblind.

Women (XX), on the other hand, have two X chromosomes, so they would need two copies of the colorblind gene to exhibit color blindness—one from each parent. This is much rarer since it requires both the mother to be a carrier or affected and the father to be affected or a carrier as well.

This means that the mother carries the colorblind allele on one of her X chromosomes, and since sons inherit their X chromosome from their mother and their Y chromosome from their father, they will be color-blind if they inherit the X chromosome carrying the gene.

Daughters receive one X chromosome from each parent. Since the father is not color-blind, he provides a normal vision allele via his X chromosome, which means the daughter will not be colorblind but may be a carrier if she inherits the colorblind allele from her mother.

  • The sex chromosomes of a male are XY, and those of a female are XX.
  • Color blindness is X-linked recessive, affecting males more than females.
  • A son cannot inherit colorblindness from his father since he inherits the Y chromosome from him, not an X. The correct option is 2.

User Astronavigator
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