Down syndrome is a genetic disorder that occurs when a person has either a full or partial extra copy of chromosome 21. In the United States, about one in every 700 babies is born with the condition. Down syndrome is associated with eyes that slant upward, weak muscle town and short stature. Individuals with Down syndrome also experience some degree of developmental delays.
Three different types of Down syndrome exist, according to the National Down Syndrome Society (NDSS).
Trisomy 21: Normally, the body has two copies of chromosome 21. Having three copies results in the type of Down syndrome known as trisomy 21. When this happens, the additional chromosome then replicates throughout all of the body’s cells. About 95 percent of people with Down syndrome have trisomy 21.
Mosaicism: If an embryo has three copies of chromosome 21 (instead of two), but this additional genetic material is only present in some of the initial cell divisions after fertilization, mosaicism occurs. This means that some of the cells contain the normal amount of chromosomes, while others contain an extra one. This is a rare type of Down syndrome that only makes up about 1 percent of cases. According to the NDSS, mosaicism is thought to bring about less severe symptoms of Down syndrome.
Translocation: In this type of Down syndrome, part of chromosome 21 separates itself out during cell division, at which point it attaches to another chromosome. The NDSS reports that it usually attaches to chromosome 14. Approximately 4 percent of people with Down syndrome have this type.
The cause of Down syndrome remains a mystery, however certain risk factors have been associated with an increased risk for the condition. According to the NDSS, some factors seem to increase the likelihood of having a child with Down syndrome. The biggest risk factor is advanced maternal age. As a woman ages, her risk of giving birth to a child with Down syndrome increases. For example, the NDSS reports that a 28-year-old woman has about a one in 1,000 chance of having a child born with Down syndrome. However, a 35-year-old woman’s chances are one in 350, with the statistics worsening each year after (a 40-year-old woman has a one in 100 chance).
Heredity isn’t commonly associated with Down syndrome. According to the NDSS, only about one-third of cases can be traced back to a hereditary component. However, if a woman already has a child with trisomy 21 (the most common type of Down syndrome), her chances of having another baby with the same condition is about one in 100 until the age of 40.
For children with Down syndrome, a variety of interventions are available to families. According to the National Institute of Child Health and Human Development (NICHD), the following treatment approaches can have a positive impact.