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Down Syndrome (Trisomy 21):

In humans, when a sperm fertilizes an egg, an individual will inherit one chromosome pair from the mother's egg and one from the father's sperm resulting in 23 pairs of chromosomes or 46 in total for each individual. In Down syndrome, the affected individual will have a triplication of chromosome 21 (Trisomy 21). The condition can be the result of a developing egg or sperm cell that may divide incorrectly, causing an egg or sperm cell to have an extra chromosome number 21. When this cell joins with a normal egg or sperm cell, the resulting embryo has 47 chromosomes instead of 46. If this occurs, the affected individual will have three number 21 chromosomes instead of two. This type of error in cell division causes about 95% of the cases of Down syndrome.
In about 3-4% of Down syndrome cases, a part of chromosome 21 breaks off during cell division and becomes attached to another chromosome in the egg or sperm cell. This can result in affected individuals having two normal copies of chromosome 21, plus extra chromosome 21 material, which is attached to another chromosome. The resultant abnormality is called Translocation Down syndrome.
It not clear precisely how extra genes from chromosome 21 can cause Down syndrome. Several investigators have suggested that the increased presence of specific genes may impair the interaction between various genes with some becoming more active while others are less active than normal. This alteration in the interaction among genes may lead to developmental changes to the body and may explain why some individuals with Down syndrome are more severely affected than others.