# DBT JRF 2020 — Question 11

Exam: DBT JRF 2020 · Section: B · Topic: Genetics · Difficulty: Hard

## Question

Fragile X syndrome is caused by a fragile site at the end of the long arm of X chromosome. Such a disorder is

## Options

- **A.** dominant
- **B.** all the given options
- **C.** X-linked
- **D.** caused by loss-of-function of FMR1 gene

## Answer

Option B

## Explanation

Fragile X syndrome (FXS) is the most common inherited cause of intellectual disability. Molecular basis: Expansion of a CGG trinucleotide repeat in the 5' UTR of the FMR1 gene on the X chromosome (Xq27.3). Normal: <55 repeats; Premutation: 55–200 repeats; Full mutation: >200 repeats → hypermethylation of FMR1 promoter → gene silencing → loss of FMRP (Fragile X Mental Retardation Protein), an RNA-binding protein regulating synaptic mRNA translation. FXS is X-linked (males hemizygous are more severely affected), caused by loss-of-function of FMR1, and considered dominant in expression (heterozygous females show partial phenotype). All three options in the question are aspects of FXS.

Source: https://mypyqbuddy.com/questions/fragile-x-syndrome-is-caused-by-a-fragile-6a05a8a02adafec470293223