Read the paragraph and answer the question.

Broadly, genetic disorders may be grouped into two categories - Mendelian disorders and Chromosomal disorders. 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 trait may also be linked to the sex chromosome as in case of haemophilia. It is evident that this X-linked recessive trait shows transmission from carrier female of male progeny.

What is the probability of having a haemophilic child to a carrier mother and a normal father?

25%

The correct answer is Option (4) -25%

Haemophilia : This sex linked recessive disease, which shows its transmission from unaffected carrier female to some of the male progeny has been widely studied. In this disease, a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected. Due to this, in an affected individual a simple cut will result in non-stop bleeding. The heterozygous female (carrier) for haemophilia may transmit the disease to sons. The possibility of a female becoming a haemophilic is extremely rare because mother of such a female has to be at least carrier and the father should be haemophilic (unviable in the later stage of life). The family pedigree of Queen Victoria shows a number of haemophilic descendents as she was a carrier of the disease.

The probability of having a hemophilic child when the mother is a carrier (heterozygous) for the hemophilia gene and the father is normal can be calculated using Punnett squares.

In this case, the mother's genotype is X X^h (where X represents the normal allele and X^h represents the hemophilic allele), and the father's genotype is XY

The possible combinations for the offspring are as follows:

1. X X (normal)
2. X X^h (carrier)
3. X^h Y (hemophilic)
4. XY (normal)

So, out of the four possible combinations, one of them (X^hY) represents a hemophilic child. Therefore, the probability of having a hemophilic child in this scenario is 1/4 or 25%.