Robertsonian translocation

Example of a Robertsonian translocation in which the long arms of 1 chromosome 14 and 1 chromosome 21 are fused in the carrier parent (upper panel) is a cause of Down syndrome in the offspring. One of the 3 viable gametes will be normal, 1 will have a balanced rearrangement, and 1 will contain the fused chromosome [der(14;21)] as well as the unaffected chromosome 21. Normal fertilization of this gamete results in a fetus with trisomy 21. Other possible segregation products are gametes lacking a chromosome 21, gametes lacking a chromosome 14, and gametes with 1 chromosome 14 and a derivative chromosome der(14;21), all of which are not viable.