Down Syndrome Home > Causes of Down Syndrome
From what is known about the causes of Down syndrome, environmental factors and parents' behavioral activity do not seem to play a role. Most cases (approximately 92 percent) are caused by trisomy 21, which means each cell in the body has three copies of chromosome 21 instead of the usual two copies. These Down syndrome causes, while genetic in nature, are not hereditary in the usual sense; the exception is translocation Down syndrome.
The causes of Down syndrome are genetic problems related to chromosome 21. While Down syndrome is genetic in that it involves an extra chromosome 21, it is not usually hereditary in the conventional sense. That is, Down syndrome is not inherited; it does not "run in families." Down syndrome is usually the result of a random event that occurs during the formation of reproductive cells (eggs and sperm). One exception is Down syndrome caused by a balanced translocation (see below).
As far as we know, Down syndrome is not caused by any behavioral activity of the parents or environmental factors.
Chromosomes, the spaghetti-like strands of hereditary material found in each cell of the body, determine such characteristics as the color of our eyes and hair, our height, and whether we are male or female. In humans, there are 23 pairs of chromosomes in each cell. A person inherits one copy of each chromosome from each parent.
Each cell contains 22 pairs of chromosomes (called autosomes) that are the same in males and females. The remaining pair of chromosomes, the X- and Y-chromosomes, are not shaped similarly, and thus are not matched in the same way as the autosomes. The X- and Y-chromosomes are called sex chromosomes. They are responsible for the difference in development between males and females. A Y-chromosome contains genes responsible for testis development; and the presence of an X-chromosome paired with a Y-chromosome will determine male development. On the other hand, two X-chromosomes are required for normal ovarian development in females.