Question

The karyotype of a young girl who is affected with familial Down syndrome revealed a total of 46 chromosomes. Her older brother, however, who is phenotypically unaffected, actually had 45 chromosomes. Explain how this could happen. What would you expect to be the numbers of chromosomes in the parents of these two children?

Short answer

The girl has 46 chromosomes because one of her chromosome 21 is attached to chromosome 14 due to translocation, contributing to her familial Down Syndrome. The unaffected brother has 45 chromosomes, because he inherited the chromosome 14 with the translocated chromosome 21, hence he is a carrier without symptoms. The parents, being carriers, also likely have 45 chromosomes each.

Step-by-step solution

Understanding Familial Down Syndrome

Familial Down Syndrome arises due to a chromosomal translocation wherein a part of the chromosome 21 attaches to another chromosome. In this case, it's the typical chromosome 14. Thus instead of having a normal pair of chromosome 21, the affected individual ends up with three copies of genetic material from chromosome 21.

Chromosome Number in the Girl with Down Syndrome

Even though the girl with familial Down Syndrome has an extra copy of genetic material from chromosome 21, her total chromosome number remains 46. This is because, rather than having three separate chromosome 21, one of these chromosomes is attached to chromosome 14 due to translocation.

Chromosome Number in the Unaffected Brother

The unaffected brother has 45 chromosomes because he possesses one normal chromosome 21, one normal chromosome 14, and one chromosome 14 with translocated chromosome 21. This form of arrangement still makes him a carrier for familial Down Syndrome while not being affected himself.

Chromosome Number in the Parents

The parents are most likely carriers of the translocation, which means they also have 45 chromosomes each: 22 normal pairs, one normal chromosome 21, one normal chromosome 14, and one chromosome 14 with translocated chromosome 21. Being carriers they can pass on this translocated chromosome to their children.