Down syndrome is the most common chromosomal anomaly, occurring once in every 733 births.
During normal cell development, the original cell begins to grow by dividing and duplicating itself. Sometimes, for reasons that are not yet understood, the original cell does not divide evenly. When the extra genetic material is located at the 21st chromosome, it is called trisomy-21, which is also known as Down syndrome, after John Landgon Down, an English physician who first described the condition in 1866.
Although babies with Down syndrome have extra genetic material at the number 21 chromosome, all of their other chromosomes are normal. In fact, the material in the number 21 chromosome is normal, too—there is just more of it.
There is great diversity regarding intelligence, learning styles, physical ability, creativity and personality, because of the influence of the other 46 chromosomes in each baby’s genetic blueprint.
There are 3 types of Down syndrome, and Avery’s is the most common, which comprises 95% of the diagnoses. It is called nondisjunction trisomy-21, which means chromosome 21 did not disjoin from itself and divide evenly. This happens at the beginning of cell division and the extra genetic material is copied in each of his cells.
There is another form called translocation trisomy-21, where part of the number 21 chromosome breaks off and attaches itself elsewhere, sometimes to the number 14 chromosome, or sometimes to the other number 21 chromosome.
The third type of Down syndrome is a rare form called mosaicism, in which the trisomy occurs a bit later in cell division, so only some of the cells contain and perpetuate it.
The current, preferred terminology is Down syndrome. A child is a child first, so instead of a “Down’s baby,” you would say “a baby with Down syndrome.” This phrasing is called People First language and applies to anyone with a learning difference or a physical difference.
People with Down syndrome are not severely retarded, but fall into the mild to moderate range.
People with Down syndrome are not always happy (they have a full range of feelings, like everyone else).
Down syndrome is not fatal, and 80% of adults with Down syndrome live to age 55 or beyond.
Down syndrome is part of more than 350,000 families in the United States. It occurs in all races, and at all socio-economic levels.