Down syndrome is a genetic condition caused by extra genes from the 21st chromosome that result in certain characteristics including some degree of cognitive disability, and other developmental delays. The incidence of Down syndrome in the United States is about 1 in 1,000 births. There is no association between Down syndrome and culture, ethnic group, socioeconomic status or geographic region.
Age Related Risks
Generally, the chance of having a Down syndrome birth is related to the motherâ€™s age. The odds of having a child with Down syndrome at age 35 are about 1 in 350. Under age 25, the odds are about 1 in 1,400. At age 40, the odds are about 1 in 100.
Types of Down Syndrome
There are three types of Down syndrome:
- Trisomy 21 â€” An estimated 95 percent of people with Down syndrome have Trisomy 21, meaning an individual has three instead of two 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 womanâ€™s or a manâ€™s pair of chromosomes normally split so that only one chromosome is in each egg or sperm. In Trisomy 21, the 21st chromosome pair does not split and a double-dose goes to the egg or sperm. An estimated 95 percent to 97 percent of the extra chromosome is of maternal origin.
- Translocation â€” This occurs in about 3 percent to 4 percent of people with Down syndrome. In this type, an extra part of the 21st chromosome gets stuck onto another chromosome. In about half of these situations, one parent carries the extra 21st chromosome material in a â€œbalancedâ€ or hidden form.
- Mosaicism â€” In mosaicism, the person with Down syndrome has an extra 21st chromosome in only some of the cells but not all of them. The other cells have the usual pair of 21st chromosomes. About 1 percent to 2 percent of people with Down syndrome have this type.
In addition to developmental delays, some common physical traits are an upward slant of the eyes; flattened bridge of the nose; single, deep crease on the palm of the hand; and decreased muscle tone. A child with Down syndrome, however, may not have all these symptoms.
Typically, parents of Down syndrome babies donâ€™t have any symptoms themselves even though they may carry an abnormal gene.
Risk of Recurrence
After the birth of a child with Down syndrome, the risk of having a second child with Down syndrome depends on what caused the condition in the first child. A medical geneticist or genetic counselor should be consulted to get more precise information on the risks. The following are some general guidelines:
- Trisomy 21 â€” The risk of Down syndrome recurring is either 1 percent to 2 percent or the risk based on the motherâ€™s age, whichever is higher.
- Translocation â€” The risk of Down syndrome recurring depends on if the translocation was inherited or not inherited. If not inherited, the risk is the same as the risk based on the motherâ€™s age. If inherited, the risk depends on the type of translocation and the sex of the parent carrying the chromosome abnormality. For the more common â€œbalancedâ€ or hidden translocation, the risk ranges from about 3 percent when the father is the carrier to about 12 percent when the mother is the carrier. If a parent carries a rare â€œ21;21â€³ translocation, the risk for Down syndrome is 100 percent.
- Mosaicism â€” The risk is about the same as risk based on the motherâ€™s age.