Q.19 Hypophosphatemia is manifested by an X-linked dominant allele.
What proportion of the offsprings from a normal male and an affected
heterozygous female will manifest the disease?
- ½ sons and ½ daughters
- all daughters and no sons
- all sons and no daughters
- ¼ daughters and ¼ sons
HypophosphatemiaX-Linked Dominant: Offspring Disease Proportion Explained
Hypophosphatemia follows X-linked dominant inheritance, where a heterozygous affected female (XHXh) and normal male (XhY) produce offspring with ½ sons and ½ daughters manifesting the disease. This pattern arises because sons inherit the mother’s X chromosomes equally, and daughters express the dominant allele from her.
Correct Answer
The correct option is ½ sons and ½ daughters. In X-linked dominant inheritance, the heterozygous female carries one disease allele (XHXh), passing it to 50% of sons (who get XHY and express the trait due to hemizygosity) and 50% of daughters (who get XHXh and express it due to dominance). Sons receiving XhY remain normal, as do daughters with XhXh.
Punnett Square Analysis
Cross: Female XHXh × Male XhY
| Xh | Y | |
|---|---|---|
| XH | XHXh (affected daughter) | XHY (affected son) |
| Xh | XhXh (normal daughter) | XhY (normal son) |
This yields 50% affected offspring overall: half the sons and half the daughters. Males cannot transmit to sons, but affected mothers transmit equally to both sexes.
Option Explanations
- ½ sons and ½ daughters: Correct, as shown in the Punnett square; equal transmission probability.
- All daughters and no sons: Incorrect; applies if the father is affected (passes XH to all daughters, Y to sons). Here, the mother is affected and heterozygous.
- All sons and no daughters: Incorrect; contradicts X-linked pattern, as sons get mother’s X (50% chance) and daughters cannot all be unaffected.
- ¼ daughters and ¼ sons: Incorrect; implies recessive inheritance or smaller litter, not matching dominant X-linked 50% transmission.
Genetic Implications
X-linked dominant disorders like hypophosphatemia affect both sexes but show distinct patterns: no male-to-male transmission, and affected females pass to half offspring regardless of sex. This differs from autosomal dominant (equal across sexes) or X-recessive (mostly males). Understanding aids genetic counseling.
