Which of the following is incorrect regarding colour blindness?

The correct answer is Option (2) → It is a dominant disease

Colour blindness is a sex-linked recessive Mendelian disorder caused by a defect in the red or green cone of the eye. Because the gene is recessive, a carrier woman with one normal X is not colour blind herself. It is far more common in males, who have only one X chromosome.

Option (1) Occurs in about 8 per cent of males and only about 0.4 per cent of females — Correct about colour blindness. The book gives these figures: it occurs in about 8 per cent of males and only about 0.4 per cent of females.

Option (2) It is a dominant disease — Incorrect about colour blindness. Colour blindness is not dominant. A carrier mother does not show it because her normal dominant gene suppresses the defective recessive one.

Option (3) It is a recessive disease — Correct about colour blindness. The gene is recessive, so a female shows the disease only when both her X chromosomes carry it.

Option (4) It is a Mendelian disorder — Correct about colour blindness. The book lists colour blindness among the most common Mendelian disorders, along with haemophilia, sickle-cell anaemia and phenylketonuria.

NCERT Extract:
Mendelian Disorders – “… Most common and prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle-cell anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc.”
Mendelian Disorders – “… Colour Blidness : It is a sex-linked recessive 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. … The mother is not herself colour blind because the gene is recessive.”