Down syndrome
Definition
Down syndrome is the most common chromosome disorder and genetic cause of mental retardation. It occurs because of the presence of an extra copy of chromosome 21. For this reason, it is also called trisomy 21.
Description
When a baby is conceived, the sperm cell from the father and the egg cell from the mother undergo a reduction of the total number of chromosomes from 46 to 23. Occasionally an error occurs in this reduction process and instead of passing on 23 chromosomes to the baby, a parent will pass on 24 chromosomes. This event is called nondisjunction and it occurs in 95% of Down syndrome cases. The baby therefore receives an extra chromosome at conception. In Down syndrome, that extra chromosome is...
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