Final answer:
Down syndrome is characterized by having three copies of chromosome 21, known as trisomy 21, caused by nondisjunction during meiosis. The risk of giving birth to a child with Down syndrome increases with maternal age, particularly after age 36.
Step-by-step explanation:
An individual with Down syndrome has three copies of chromosome 21. This condition is also known as trisomy 21 and is a result of nondisjunction during meiosis, which is a failure of chromosome pairs to separate properly. The presence of an extra chromosome causes various physical and intellectual disabilities. It is one of the most common chromosome abnormalities that can persist through adulthood.
A problem with the spindle checkpoint during cell division can lead to nondisjunction because the spindle fibers are responsible for the accurate separation of chromosomes. If the checkpoint fails, chromosomes might not divide equally, resulting in one cell with an extra chromosome and another with a missing chromosome.
As maternal age increases, particularly in women over 36, the risk of a nondisjunction event and having a child with Down syndrome increases. The paternal age is less significant because nondisjunction is less likely to happen in sperm compared to eggs.