Hemophilia is a sex-linked recessive trait that is carried on the X chromosome. Women have two X chromosomes (XX), while men have one X and one Y chromosome (XY).
Since the woman is a carrier for hemophilia, her genotype is X^H X^h, where X^H is the normal allele and X^h is the hemophilia allele. The man, being normal, has the genotype X^H Y.
When they have a son, he will inherit one X chromosome from his mother and one Y chromosome from his father. The possible combinations for their son’s X chromosome from the mother could be either X^H (normal) or X^h (hemophilia). The father contributes the Y chromosome.
The potential genotypes for their son are:
- If he receives X^H from his mother and Y from his father: X^H Y (normal, not affected).
- If he receives X^h from his mother and Y from his father: X^h Y (has hemophilia).
Here’s the breakdown of probability:
- Probability of inheriting the normal X (X^H) from the mother = 50%
- Probability of inheriting the hemophilia allele (X^h) from the mother = 50%
Thus, the probability that their son will have hemophilia is 50%.
The correct answer is: 50%.
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