What is the genetic cause of Down syndrome?

Down syndrome, also known as trisomy 21, is primarily caused by the presence of an extra copy of chromosome 21. This genetic anomaly occurs due to nondisjunction during meiosis, where chromosomes fail to separate properly, resulting in a gamete with an additional chromosome. When this gamete fuses with a normal gamete, the resulting embryo has three copies of chromosome 21 instead of the usual two.

The characteristics associated with Down syndrome stem from this additional genetic material, which affects normal development and leads to various phenotypic features and health concerns. Therefore, trisomy 21 is the definitive genetic condition responsible for Down syndrome, making it the correct answer to the question.

The other choices represent different genetic conditions. Deletion of chromosome 21q does not cause Down syndrome and is associated with different syndromes. Trisomy 18 and trisomy 13 refer to Edwards syndrome and Patau syndrome, respectively, which are distinct genetic disorders caused by the presence of extra chromosomes 18 and 13, and do not relate to Down syndrome.