The reason for occurrence of Down syndrome in a child if chromosomal translocation occurs in gonads of the child’s parents, involving attachment of third copy of chromosome 21 to chromosome 14. Concept introduction: Down syndrome is caused by chromosomal aberration in which trisomy of chromosome 21 occurs. The parents of the children with Down syndrome are normal. Chromosomal translocation is a process in which a part of chromosome does not break during meiosis and is carried to the gamete as it is.

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Chapter 15.4, Problem 1CC

Summary Introduction

To determine: The reason for occurrence of Down syndrome in a child if chromosomal translocation occurs in gonads of the child’s parents, involving attachment of third copy of chromosome 21 to chromosome 14.

Concept introduction:

Down syndrome is caused by chromosomal aberration in which trisomy of chromosome 21 occurs. The parents of the children with Down syndrome are normal.

Chromosomal translocation is a process in which a part of chromosome does not break during meiosis and is carried to the gamete as it is.

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About 5% of individuals with Down syndrome havea chromosomal translocation in which a third copy ofchromosome 21 is attached to chromosome 14. If thistranslocation occurred in a parent’s gonad, how could itlead to Down syndrome in a child?

In humans, chromosome 16 sometimes has a heavily stained area in the long arm near the centromere. This feature can be seen through the microscope but has no effect on the phenotype of the person carrying it. When such a “blob” exists on a particular copy of chromosome 16, it is a constant feature of that chromosome and is inherited. A couple conceived a child, but the fetus had multiple abnormalities and was miscarried. When the chromosomes of the fetus were studied, it was discovered that it had three copies of chromosome 16 (it was trisomic for chromosome 16), and that two of the three chromosome 16s had large blobs. Both chromosome 16 homologs in the mother lacked blobs, but the father was heterozygous for blobs. Which parent experienced nondisjunction, and in which meiotic division did it occur?