These being 44 autosomes and two sex chromosomes. Normal reproductive cells contain one half of that number- 22 autosomes and one sex chromosome. The genetic effect associated with Down syndrome is the presence of extra material on the chromosome pair 21. This extra material in chromosome pair 21 is usually another chromosome. Translocation is an interchange of chromosomes or parts of chromosomes which may result in a mismatched pair.
Translocation occurs in about four percent of the cases.
In a few cases, a person can carry a broken chromosome 21 without showing any symptoms of Down syndrome because the correct amount of genetic material is present, just out of place. Accounting for about one percent of the cases of Down syndrome is mosaicism. Essentially this is a fraction of an extra chromosome 21 attached to one of the existing chromosomes. Being that only some of their cells have an abnormal number of chromosomes, babies with mosaic Down syndrome may have only some of the features of the disorder. Although surveys are all different the incidence of Down syndrome is around 1 in 800 live births.