Question

Which of the following individuals will have fragile X chromosome?

(a) 6 repeats of CGG

(b) 50 repeats of CGG

(c) 150 repeats of CGG

(d) 700 repeats of CGG

I have been asked this question at a job interview.

Enquiry is from Dynamic Mutation and Anticipation topic in section Nuclear Export & Post Mendelian Genetics of Cytogenetics

Answer 1

The correct choice is (d) 700 repeats of CGG

To explain: The normal number of CGG repeats in an individual is about 6-50. If a daughter has 50-200 repeats then the next generation is likely to have 200-1300 repeats leading to the disease. However, the daughter doesn’t show the defect.