Final answer:
Down syndrome is a chromosomal disorder caused by trisomy 21, where an individual has an extra chromosome 21. The disorder typically arises due to chromosomal nondisjunction during meiosis, with the risk increasing with maternal age. Most embryos with extra chromosomes do not survive, but individuals with Down syndrome can live into adulthood.
Step-by-step explanation:
The chromosomal disorder associated with Down syndrome is characterized by an individual having three copies of chromosome 21, instead of the usual two.
This condition, known as trisomy 21, leads to 47 total chromosomes in the cells of individuals with Down syndrome instead of the normal 46 (diploid state).
The extra genetic material affects the normal development and functioning of the body, leading to physical and mental disabilities.
Chromosomal nondisjunction during meiosis is the main cause of Down syndrome. This error in cell division results in a gamete with an additional chromosome.
As a woman ages, the likelihood of nondisjunction increases, which explains why the risk of having a child with Down syndrome is higher in women over 36. It is less frequent in sperms, Therefore, the age of the father is generally a less significant risk factor.
Most embryos with extra chromosomes do not survive to birth.
However, Down syndrome is an exception where individuals with the extra chromosome can survive into adulthood, although with varying degrees of impairments.