9. A mutation in an X-chromosome gene “green” results in a green beard. If a green
bearded man marries a non-carrier woman,
a. all sons will have a green beard
b. half the sons will have a green beard
c. all daughters would be carriers
d. all daughters would be non-carriers
X-Linked Green Beard Mutation Inheritance: CSIR NET Genetics MCQ Solved
A mutation in the X-chromosome “green” gene causes a green beard in affected males under X-linked recessive inheritance. When a green-bearded man (genotype X^g Y) marries a non-carrier woman (X^G X^G), sons inherit the mother’s normal X^G and father’s Y chromosome, resulting in no green beards, while all daughters become carriers (X^G X^g) with normal phenotype.
Option Analysis
Option a: All sons will have a green beard
Sons receive Y from father and X^G from mother, so genotype X^G Y expresses normal beard. This option is incorrect.
Option b: Half the sons will have a green beard
All sons get identical X^G from homozygous mother, so none show the recessive trait. This option is incorrect.
Option c: All daughters would be carriers
Daughters inherit father’s X^g and mother’s X^G, making them heterozygous carriers (X^G X^g) who do not express the trait. This option is correct.
Option d: All daughters would be non-carriers
Daughters must receive father’s mutated X^g, so all carry the allele. This option is incorrect.
Punnett Square Breakdown
| X^G (Mother) | |
|---|---|
| X^g (Father) | X^G X^g (carrier daughter) |
| Y (Father) | X^G Y (normal son) |
All daughters carry the mutation; no sons affected.
