Down syndrome develops from a genetic abnormality in cell division, known as trisomy 21, which produces a third copy of chromosome 21 in every cell, Genetics Home Reference explains. In translocation Down syndrome, a partial copy of chromosome 21 attaches to another chromosome. The rarest variant is mosaic Down syndrome, which occurs when extra copies of chromosome 21 only appear in a portion of cells. In all cases, extra genetic material interrupts healthy fetal growth and impairs development after birth.
Cells normally carry 23 chromosomal pairs, and each pair consists of a chromosome from the father's sperm and the mother's egg, according to March of Dimes. In trisomy 21 or translocation Down syndrome, an egg or sperm cell fails to divide correctly, causing it to transmit flawed genetic material during fertilization. Trisomy 21 results in 47 chromosomes, instead of 46. Translocation produces the correct number of pairs, but it still contains conflicting genetic material from chromosome 21. In contrast, mosaicism develops after fertilization.
Trisomy 21 accounts for 95 percent of Down syndrome cases, March of Dimes states. Translocation occurs in 3 to 4 percent of cases, and mosaicism only accounts for 1 to 2 percent of cases. People with Down syndrome have a wide-ranging degree of disability, but as of 2015, the average life expectancy is about 60 years of age. Common health complications of Down syndrome include heart or intestinal defects, hearing loss and susceptibility to infections.