Read the given passage and answer the following questions.
Mendelian disorders are mainly determined by alteration or mutation in the single gene. These disorders are transmitted to the offspring on the same lines as we have studied in the principle of inheritance. The pattern of inheritance of such Mendelian disorders can be traced in a family by the pedigree analysis. Most common and prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle cell anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc. It is important to mention here that such Mendelian disorders may be dominant or recessive.
Why are females less likely to be colour blind compared to males?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Females have two X chromosomes, so the normal gene on one X chromosome can suppress the effect of the recessive gene on the other X chromosome
Colour blindness is a sex-linked recessive disorder. Its gene lies on the X chromosome. A male (XY) has only one X chromosome, so a single recessive allele is enough to make him colour blind. A female (XX) is colour blind only when both her X chromosomes carry the recessive allele. That is why the disorder is far more common in males.
Option (1) Females have two X chromosomes, so the normal gene on one X chromosome can suppress the effect of the recessive gene on the other X chromosome — Correct. With two X chromosomes, a female who has one recessive allele usually also has a dominant normal allele on the other X. The normal allele hides the effect of the recessive one, so she is only a carrier.
Option (2) Females have a stronger immune system — Incorrect. The immune system protects the body against pathogens. Colour blindness is not an infection; it is caused by a gene on the X chromosome, so immunity has no part in it.
Option (3) Females have fewer cone cells in their eyes — Incorrect. The defect lies in the red or green cone of the eye because of a mutant gene. The number of cone cells does not decide whether a person is colour blind.
Option (4) Females have a different visual cortex — Incorrect. The disorder starts in the cones of the eye, which fail to tell red from green. The difference between males and females comes from the number of X chromosomes, not from the brain.
NCERT Extract:
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. 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. That means that its effect is suppressed by her matching dominant normal gene.”