What are the types of Downs Syndrome?
About 95% of people with Down’s syndrome have Trisomy 21, ie. 3 instead of 2 number 21 chromosomes. We normally have 23 pairs of chromosomes, each made up of genes. During the formation of the egg (or the sperm) a woman’s (or a man’s) pair of chromosomes normally split so that only one chromosome is in each egg (or sperm). In Trisomy 21, or nondisjunction, the twenty-first chromosome pair does not split and a double-dose goes to the egg (or sperm). It is estimated that 95-97% of the extra chromosome is of maternal origin. The second type of Down’s Syndrome is known as translocation. It occurs in about 3-4% of people with Down’s Syndrome. In this type, an extra part of the twenty-first chromosome gets stuck onto another chromosome. In about half of these situations, one parent carries the extra twenty-first chromosome material in a “balanced”, or hidden, form. The third type of Down’s Syndrome is called mosaicism. In mosaicism, the person with Down’s syndrome has an extra twenty-first c
