Final answer:
Down syndrome, or trisomy 21, is caused by an abnormal number of chromosomes. It is not caused by gene mutations. Nondisjunction during meiosis, where chromosomes fail to separate properly, is the most common cause of Down syndrome.
Step-by-step explanation:
Down syndrome, also known as trisomy 21, is caused by having three copies of chromosome 21 instead of the usual two copies. This genetic disorder is not caused by gene mutations, but by a chromosomal abnormality where there is an extra copy of chromosome 21.
The most common cause of Down syndrome is chromosomal nondisjunction during the formation of reproductive cells. Nondisjunction occurs when chromosomes fail to separate properly during meiosis, resulting in the abnormal number of chromosomes in the resulting offspring.
It is important to note that Down syndrome is not caused by gene mutations, but rather by errors in chromosomal division during reproductive cell formation.