Down syndrome (DS) is a disorder that results from an error in the cell division called nondisjunction.
In this situation, homologous chromosomes fail to separate properly during reduction division of meiosis.
As a result, an extra chromosome passes to one of the daughter cells (gametes). Individuals with the disorder usually have 47 chromosomes instead of the normal 46 (an extra chromosome 21) For this reason Down syndrome is also called trisomy 21.
Down syndrome is characterized by mental retardation, retarded physical development (short stature and stubby fingers), distinctive facial structures (large tongue, flat profile, broad skull, slanting eyes and round head), and malformation of the heart, ears, hands and feet.
Sexual maturity is rarely attained.
An estimated 1 in 600 to 800 infants is born with Down syndrome and a women's chances of giving birth to a child with the disorder increase with age.
For example - the chance of having a baby with this syndrome is 1 in 1205 for a 25 year-old women, but is 1 in 365 for 35 year-old woman.
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