Named after physician John Langdon Down, Down Syndrome (DS) is a genetic disorder that is caused by an extra chromosome 21 that is present in all or some of the individual’s cells. There are three types of chromosome abnormalities in Down syndrome. The first is called trisomy 21 and is the most common form of DS. With trisomy 21, the individual has an extra chromosome 21, which results in a total of 47 chromosomes in each cell rather than the typical 46. The second is called mosaicism and is the least common form of DS. In this case, only some of the cells contain 47 chromosomes, while the other cells contain an accurate 46. The third type is called translocation where the extra chromosome attaches (or translocates) to a different chromosome, such as 14 or 22.
The additional chromosome is almost always the result of faulty cell division, in either the egg or sperm, resulting in 3 chromosome 21s (2 from the egg and 1 from the sperm, or vice-versa). Though the extra chromosome could come from either the father or the mother, only 5% of the cases have been traced to the father. Women age 35 years and older have a significantly increased chance of having a child with DS, however, 80% of children born with DS are actually born to mothers under the age of 35 years due to higher fertility rates.
DS is one of the most common chromosome abnormalities. It is estimated that the incidence is between 1 in 800 to 1,000 live births. There are approximately 350,000 individuals in the United States with DS. It occurs in all racial, ethnic, and socioeconomic groups. Recent research shows that slightly more males than females are born with DS in comparison with the general population.
Down syndrome was first accurately described in 1866 by an English physician named John Langdon Down. Then, in 1959, a French physician named Jerome Lejeune identified Down syndrome as a chromosome anomaly. It was Lejeune who observed 47 chromosomes in individuals with DS, rather than 46 chromosomes.
The cause of DS is not known, however, some experts believe that hormonal abnormalities, x-rays, viral infections, immunological problems, or genetic predisposition may be the cause of improper cell division, resulting in the syndrome. What is known, however, is that the extra chromosome 21 causes an extra dose of some proteins, which causes the typical features of the syndrome. In addition, it should be noted that DS is not the result of something that either parent did or did not do.
Generally, when a child is born with Down syndrome, the appearance of the child is the first indication that he or she may have this disability. Therefore, a chromosome analysis is conducted which looks for three chromosome 21s or other gene variants. It can also be diagnosed prenatally by either an amniocentesis or chorionic villus sampling.
What to Expect
Children with DS can usually do what “typical” children can do, such as walk, talk, play, dress themselves, potty train, etc., however, in general these skills develop later than other children. Individuals with Down syndrome should be a part of the community; they strive to accomplish goals just like everyone else. They go to school, have jobs and enjoy leisure activities. Organizations, (i.e., banks, corporations, restaurants, entertainment firms, and computer organizations) actively seek young adults with DS for employment. People with DS bring to their jobs enthusiasm, reliability, and dedication.
Many adults with DS form relationships with others and some even marry. Women with Down syndrome are fertile and can have children; men are believed to be sterile. The life expectancy of an individual with Down syndrome is traditionally below average.