Why is red-green colour blindness found more often in males than in females?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The gene is present on the X chromosome, and males have a single X chromosome.
Red-green colour blindness is a sex-linked recessive disorder: the genes that lead to it are on the X chromosome. A male has one X chromosome, so a single defective gene on it makes him colour blind. A female has two X chromosomes, and the normal gene on her other X usually hides the defective one, so she is only a carrier. This is why the defect is far more common in males.
Option (1) The gene is present on the X chromosome, and males have a single X chromosome. — Correct. The gene is X-linked, and with only one X a male has no second, normal copy to mask the defect.
Option (2) The gene is present on the Y chromosome, which a son receives from his father. — Incorrect. The Y chromosome does not carry the gene for colour blindness. A son gets his Y from his father, but colour blindness reaches him through his mother’s X.
Option (3) The gene is dominant and present on the X chromosome, so one copy is enough to cause the defect. — Incorrect. The gene is X-linked but recessive, not dominant. A dominant X-linked gene would in fact show up more often in females, who have two X chromosomes.
Option (4) A son receives his X chromosome from his father, who passes the defective gene to him. — Incorrect. A son receives his X chromosome from his mother; from his father he receives the Y. That is why the son of a carrier woman has a 50 per cent chance of being colour blind.
NCERT Extract:
Mendelian Disorders – “… 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. 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.”