In familial Down syndrome resulting from Robertsonian translocation,
A)the affected individual has three copies of the complete chromosome 21.
B)the affected individual is missing a copy of chromosome 21.
C)the affected individual has three copies of the long arm of chromosome 21,one of which is attached to part of chromosome 14.
D)the affected individual has two copies of the long arm of chromosome 21,one of which is attached to part of chromosome 14.
E)the affected individual has three copies of the long arm of chromosome 14,one of which is attached to chromosome 21.

Question

Quizplus · Accepted Answer

Robertsonian translocation occurs when two non-homologous chromosomes break and exchange fragments. In familial Down syndrome resulting from Robertsonian translocation, a piece of chromosome 21 attaches to a piece of chromosome 14, resulting in an individual with three copies of the long arm of chromosome 21 but still having a total of 46 chromosomes (i.e., no missing or extra chromosomes). This individual will still have Down syndrome because they have three copies of the critical genes on chromosome 21. Therefore, the best choice is C: the affected individual has three copies of the long arm of chromosome 21, one of which is attached to part of chromosome 14.

In Familial Down Syndrome Resulting from Robertsonian Translocation