Question

Familial Down syndrome is similar to primary Down syndrome in that it is caused by trisomy 21. However, in familial Down syndrome, all or part of the third copy of chromosome 21 has translocated onto another autosome, typically chromosome 14. In cases of familial Down syndrome, one of the parents of the affected child is often a carrier of a translocated chromosome. The translocation carrier parent does not have Down syndrome because he or she has a total of two copies of chromosome 21. Suppose that a woman is a translocation carrier of chromosome 21 on chromosome 14. She conceives six zygotes with a normal man who carries no aneuploidies. Match the description of the chromosomes inherited by each zygote with the viability of the zygote.

Viable normal, or translocation carrier Viable, with Down syndrome Inviable two normal copies of 21, one normal copy of 14 one normal copy of 21, one 21 to 14 translocation, two normal copies of 14 one normal copy of 21, one 21 to 14 translocation, one normal copy of 14 two normal copies of 14, one normal copy of 21, one normal copy of 14 two normal copies of 21 two normal copies of 4 oo malcois calion one 21 to 14 translocation

Answer 1

Viable normal, or translocation carrier

one normal copy of 21, one 21 to 14 translocation, one normal copy of 14

This is known as balanced translocation and individuals with such chromosomal arrangement have 45 chromosomes and are phenotypically normal.

Viable, with Down syndrome

one normal copy of 14 two normal copies of 21 one 21 to 14 translocation

It is trisomy of chromosome 21 (three copies of 21 chromosome).

Inviable

two normal copies of 21, one normal copy of 14

It is called monosomy 14 and it is lethal.

one normal copy of 21, one 21 to 14 translocation, two normal copies of 14

It is called trisomy 14 and it is lethal.

two normal copies of 14, one normal copy of 21,

It is called monosomy 21 and it is also lethal.