Read the given passage and answer the following questions:
Colour blindness is a sex-linked disorder due to defect in either red or green cone of eye resulting in failure to discriminate between red and green colour. This defect is due to mutation in certain genes present in the X chromosome. It occurs in about 8 per cent of males and only about 0.4 per cent of females. This is because the genes that lead to red-green colour blindness are on the X chromosome.
Which of the following is not possible in the case of colour blindness?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Diseased father to pass gene to his son.
The gene for colour blindness lies on the X chromosome. A mother passes one of her two X chromosomes to every child, son or daughter. A father passes his X chromosome only to his daughters and his Y chromosome to his sons. So an X-linked gene can never go from a father to his son.
Option (1) Carrier mother to pass gene to her son. — Possible. A carrier mother (XcX) gives her Xc to half of her sons. The son of a carrier woman has a 50 per cent chance of being colour blind.
Option (2) Carrier mother to pass gene to her daughter. — Possible. The carrier mother gives her Xc to half of her daughters too. Such a daughter becomes a carrier.
Option (3) Diseased father to pass gene to his daughter. — Possible. A colour-blind father (XcY) gives his only X chromosome, the Xc, to every daughter.
Option (4) Diseased father to pass gene to his son. — Not possible. A son receives the Y chromosome from his father and his X from his mother. The father’s Xc never reaches a son.
NCERT Extract:
Mendelian Disorders – “… Males have only one X chromosome and females have two. The son of a woman who carries the gene has a 50 per cent chance of being colour blind. The mother is not herself colour blind because the gene is recessive. That means that its effect is suppressed by her matching dominant normal gene. A daughter will not normally be colour blind, unless her mother is a carrier and her father is colour blind.”
Fertilisation and Implantation – “… After fusion of the male and female gametes the zygote would carry either XX or XY depending on whether the sperm carrying X or Y fertilised the ovum. The zygote carrying XX would develop into a female baby and XY would form a male.”