Question

Foe fragile X chromosome which of the following is wrong?

(a) The father having 50 nt repeats or above leads to disease in daughter

(b) Daughter has 50-200 repeats

(c) The grandson has 200-1300 repeats

(d) The grandson shows the disease phenotype

This question was posed to me in an online quiz.

This key question is from Dynamic Mutation and Anticipation in portion Nuclear Export & Post Mendelian Genetics of Cytogenetics

Answer 1

Right answer is (a) The father having 50 nt repeats or above leads to disease in daughter

For explanation: The father having a repeat beyond a particular threshold say 50 will pass the gene to daughter. In the daughter due to anticipation the gene could be 50-200 times repeated. Only in the next generation number of repeat will be 200-1300 and they will show the phenotype.