What are the types of Down Syndrome?
A About 95% of people with Down 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 Syndrome is known as translocation. It occurs in about 3-4% of people with Down 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 Syndrome is called mosaicism. In mosaicism, the person with Down Syndrome has an extra twenty-first chromosome in only some of the cells but not all of them. The other cells have the usual pair of twenty-first chromosomes. One to 2 percent of people with Down Syndrome have this type.(Cooley & Graham, 1991)
What is the incidence of Down Syndrome?
A The incidence
of Down Syndrome has been variously reported as 1 in 800 live births to 1
in 1,100 live births. A recent estimate in the United States puts the incidence
at about 1 in 1,000. There is no association between Down Syndrome and any
given culture, ethnic group, socioeconomic status or geographic region. (CDC,
1994) (Thompson, McInnes, & Willard, 1991)
What are a couple's chances of having a child with Down Syndrome?
A In the usual circumstance, the chances depend upon the age of the mother. The odds of having a child with Down Syndrome at age 35 are approximately 1 in 350. Under age 25, the odds are about 1 in 1400 and at age 40 the odds are about 1 in 100. (Thompson, et al., 1991) The chances of a parent of a child with Trisomy 21 having another child with Down Syndrome is approximately 1 in 100. If the child has a translocation, the recurrence risk can be as high as 100% or as low as 2%. Parents of children with translocation type of Down Syndrome should have chromosome analysis to detect a carrier state. (Pueschel, 1990)
Can people with Down Syndrome have babies?
A The chance of a woman with Down Syndrome having a baby with Down Syndrome is theoretically 50%. There is one documented instance of fertility in a male with Down Syndrome; the mother also had Down Syndrome. The pregnancy miscarried about half-way through.
