50. Paternal grandfather is hemophilic, what is probability of his grandson to be hemophilic
(1) 1/2               (2) 1/4
(3) 1/8               (4) 0

The probability of a grandson being hemophilic when the paternal grandfather is hemophilic is 1/4 (25%). This is because hemophilia is an X-linked recessive disorder, passed from affected males to their carrier daughters, who then have a 50% chance of passing the gene to their sons.

Explanation of Options in the Question

• (1) 1/2: This is the probability that a carrier mother passes the hemophilia gene to her son. It applies if the mother is a carrier directly.

• (2) 1/4: Correct probability. The paternal grandfather passes the hemophilia gene to his daughter (the mother of the grandson). She will be a carrier. Then she has a 1/2 chance of passing the gene to her son (the grandson). So, 1×12×12=141×21×21=41.

• (3) 1/8: Incorrect. This does not follow from typical X-linked recessive inheritance in this pedigree.

• (4) 0: Incorrect. There is a risk, so zero probability is not correct.

Why the Probability is 1/4?

1. The hemophilic paternal grandfather has one X chromosome with the mutation (since males have XY).

2. He passes his X chromosome to all his daughters (the mother of the grandson), who become carriers but are phenotypically normal.

3. The daughter (carrier mother of the grandson) has two X chromosomes, one normal and one mutated.

4. Each son (the grandson) has a 1/2 chance of inheriting the mutated X chromosome from his carrier mother.

5. Thus, grandson’s probability = 1×1/2×1/2=1/4.

   Introduction:
   Hemophilia is an X-linked recessive disorder primarily affecting males. When the paternal grandfather has hemophilia, understanding the probability of the grandson inheriting this condition involves analyzing the X-linked pattern of inheritance through the mother. This article explains the genetic probability of hemophilia in the grandson with a step-by-step approach.

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   This probability calculation helps clarify the path hemophilia follows through generations in an X-linked recessive manner. The probability that the grandson will be hemophilic is 1/4 or 25% due to carrier status transmission via his mother, who inherits the defective X chromosome from her affected father.​