Gene A is normally found on chromosome number 15 in humans. If amniocentesis reveals fetal cells containing gene A on chromosome 17, but not on 15, the best explanation would be that .

A type of mutation has occurred maybe translocation of gene A from chromosome 15 to chromosome 17.

Explanation:

Since gene A which is normally localised to chromosome 15 is found on chromosome 17, it can be assumed that a type of mutation has occurred which has caused the deletion of gene A from chromosome 15 and its insertion in chromosome 17. This might have occurred through translocation of gene A from chromosome 15 to chromosome 17.

This is a form of mutation as deletion of gene A from chromosome 15 may have adverse effect on the chromosome and same goes for the insertion of gene A into chromosome 17 where it might disrupt the function of the chromosome.