Final answer:
About 95% of Down syndrome cases are caused by an extra chromosome 21, which usually originates from nondisjunction in the mother's ovum during meiosis.
Step-by-step explanation:
Trisomy 21, also known as Down syndrome, is a genetic disorder caused by the presence of an extra chromosome 21. This extra chromosome typically comes from the mother's ovum. The majority of Down syndrome cases are due to nondisjunction during meiosis in the ovum, although the exact percentage can vary. Nondisjunction is a chromosomal error that results in a gamete with an abnormal number of chromosomes. Most commonly, nondisjunction events increase with maternal age, suggesting older women have a higher likelihood of giving birth to a child with Down syndrome. Careful analysis of karyotypes, such as those shown in Figure 7.7.5, can illustrate the presence of the additional chromosome that characterizes this condition.
In Down syndrome cases, it's widely recognized that the extra chromosome typically originates from the mother. Research indicates that approximately 95% of Down syndrome cases are caused by nondisjunction in the maternal ovum. The remaining 5% can be attributed to nondisjunction in the father's sperm or to post-zygotic mitotic nondisjunction events. Understanding that Down syndrome mainly results from maternal nondisjunction is critical for both diagnosis and genetic counseling.