unknown, although it does seem to be related to advancing maternal age. What is ironic is that 80 percent of children born with Down syndrome are born to women under 35 years of age. This is because younger women have higher fertility rates. What is even more amazing is that it does not contradict the fact that the chance of having a child with Down syndrome increases with the age of the mother. Although nondisjunction can be from the father, this occurs less frequently the case. Because this error in cell division is often present in the egg prior to conception, and women are born with their life supply of eggs, it has been determined that some environmental factors may be the cause of nondisjunction. However, despite years of research, the cause of nondisjunction, is still unknown. There seems to be no connection between any type of Down syndrome and parents' activities, before or during pregnancy.Mosaicism, another less significant cause of Down syndrome, occurs when nondisjunction of the 21st chromosome takes place in one of the initial cell divisions after fertilization. When this occurs, there is a mixture of two types of cells, some containing 46 chromosomes and some containing 47. Those cells with 47 chromosomes contain an extra 21st chromosome. Because of the "mosaic" pattern of the cells, the term mosaicism is used. Mosaicism is rare, being responsible for only 1 to 2 percent of all cases of Down syndrome. Some research has shown that individuals with mosaic Down syndrome are less affected than those with trisomy 21. However, broad generalizations are not possible due to the wide range of abilities that people with Down syndrome possess. Translocation is a different type of chromosomal problem and occurs in only 3 to 4 percent of people with Down syndrome. Translocation occurs when part of the number21 chromosome breaks off during cell division and attaches to another chromosome. While the total number of chromosomes in the c...
