Question

If mother is carrier for haemophilia and father is normal, chance of their children to be haemophilic is:

• A. 50%
• B. 0%
• C. 100%
• D. 25%

Hint:

For a carrier mother and normal father, half of the sons have a chance of being haemophilic, giving an overall probability of 25%.

Answer 1

The Correct Option is D

Concept: Haemophilia is an X-linked recessive disorder. The defective gene is present on the X chromosome. Males are more commonly affected because they possess only one X chromosome.

Step 1: Representing the parental genotypes.
Carrier mother: \[ X^H X^h \] Normal father: \[ X^H Y \] where \[ X^H = \text{Normal allele} \] \[ X^h = \text{Haemophilia allele} \]

Step 2: Constructing the genetic cross.
\[ X^H X^h \times X^H Y \] Possible offspring: \[ X^H X^H \] Normal daughter \[ X^H X^h \] Carrier daughter \[ X^H Y \] Normal son \[ X^h Y \] Haemophilic son

Step 3: Calculating probability.
Out of four equally probable offspring combinations, only one is haemophilic. \[ \text{Probability} = \frac{1}{4} = 25% \] Hence, the probability that a child will be haemophilic is 25%.