Correct option is B
Explanation-
A premutation is a genetic alteration not sufficient to cause disease, but it can expand in the next generation and result in a full mutation that causes disease. Premutations are unstable and can increase in repeat number during meiosis, especially when transmitted by the mother.\
Fragile X syndrome is a genetic disorder caused by the expansion of CGG trinucleotide repeats in the FMR1 gene on the X chromosome. Parents (especially mothers) or grandparents may carry the premutation. When passed on, it may expand to a full mutation in the child → Fragile X syndrome.
It is the most common inherited cause of intellectual disability, especially in males.
Incorrect options-
Option A - Down Syndrome
This is caused by trisomy 21 (an extra copy of chromosome 21), mostly due to non-disjunction. It is not associated with premutations.
Option C - Klinefelter Syndrome
This represents a genetic condition in males with an extra X chromosome (XXY). It is caused by meiotic nondisjunction, not a premutation.
Option D - Alkaptonuria
A rare autosomal recessive metabolic disorder due to mutation in the HGD gene. It is a single-gene mutation, no premutation stage involved.
So, the correct answer is option B - Fragile X syndrome.
