e are more susceptible to developing Alzheimer's disease. Also, heart disease and leukemia are common in Down syndrome patients.
             Down syndrome occurs because there are three 21st chromosomes instead of two. About 95 percent of cases are caused by non-disjunction during the conception of the baby, called Trisonomy 21. Sometime prior to, or at conception, a pair of number 21 chromosomes, either in the sperm or the egg, fail to separate (National Down Syndrome Society). Trisonomy 21 happens in the egg 95 percent of cases and in the sperm only five percent (Ludman and Wynbrandt, page 21). Translocation is responsible for about 4 percent of cases (National Down Syndrome Society). This is when part of a number 21 chromosome breaks off during cell division and attaches to another cell. There are still only 46 chromosomes, but the extra part of the 21st chromosome causes the symptoms of Down syndrome. The remaining one percent of DS cases is caused by Mosaicism. Mosaicism is when nondisjunction takes place in one of the sex cells after fertilization (National Down's Syndrome Society). This causes a mix of cells with 46 and 47 chromosomes, so the effects are less
             ...