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Question
In humans the gene for red and green colour blindness is located on the sex chromosome. If a male is colour blind and is a child of normal visioned parents, then who among the following in his family is likely to be colour blind?
In humans the gene for red and green colour blindness is located on the sex chromosome. If a male is colour blind and is a child of normal visioned parents, then who among the following in his family is likely to be colour blind?
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Solution
The correct option is A Maternal grandfather
Colour blindness is a X-linked recessive disorder. Son gets its X-chromosome from mother; since the male is affected, he has got the Xc allele from his mother. According to question, his parents are normal which means that father does not carry the disease; presence of disease in father would have expressed due to its hemizygous genotype for X-chromosome. And mother is carrier who passed her affected allele to male. Father is normal, this rules out the disease to be inherited from paternal grandparents; options C and D are wrong. Father transmits its X chromosome to the daughters so the carrier mother have inherited her affected X chromosome from father which means that maternal grandfather’s genotype is XcY; option A is correct. If maternal grandmother was carrier; the mother of male would be diseased as she will get one affected allele from father and one from mother; but mother is carrier here, option B is incorrect. Correct answer is option A.
Colour blindness is a X-linked recessive disorder. Son gets its X-chromosome from mother; since the male is affected, he has got the Xc allele from his mother. According to question, his parents are normal which means that father does not carry the disease; presence of disease in father would have expressed due to its hemizygous genotype for X-chromosome. And mother is carrier who passed her affected allele to male. Father is normal, this rules out the disease to be inherited from paternal grandparents; options C and D are wrong. Father transmits its X chromosome to the daughters so the carrier mother have inherited her affected X chromosome from father which means that maternal grandfather’s genotype is XcY; option A is correct. If maternal grandmother was carrier; the mother of male would be diseased as she will get one affected allele from father and one from mother; but mother is carrier here, option B is incorrect. Correct answer is option A.
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