Final answer:
Trisomy 21, or Down syndrome, is a genetic disorder caused by an extra copy of chromosome 21 due to nondisjunction during meiosis. It leads to developmental and physical impairments, with a higher occurrence in children born to mothers over the age of 36. It is generally not inherited but can be in rare cases of familial genetic translocation.
Step-by-step explanation:
Trisomy 21, also known as Down syndrome, is a genetic disorder caused by an extra copy of chromosome 21. This condition is characterized by a cluster of physical and developmental symptoms due to this chromosomal abnormality. The primary genetic mutation responsible for 95% of Down syndrome cases is nondisjunction during meiosis, a process where chromosomes fail to separate properly, leading to a gamete with an abnormal number of chromosomes .In the case of Down syndrome, this results in an individual having three copies of chromosome 21 instead of the usual two copies. This extra genetic material disrupts the normal balance of gene expression and can lead to various characteristic features and health issues, including intellectual disability, heart defects, and distinctive facial features. The likelihood of a child being born with Down syndrome increases with maternal age, particularly in mothers over the age of 36.
While Down syndrome is usually not considered an inheritable condition in the traditional sense, because it typically occurs as a random event during the formation of reproductive cells, rare instances of familial Down syndrome can occur. This happens when there is a heritable genetic translocation involving chromosome 21. In such cases, a parent may carry a balanced form of the translocation without exhibiting symptoms of Down syndrome, but may pass on an unbalanced translocation leading to the syndrome in their offspring.