What causes Down Syndrome?

Down Syndrome is primarily caused by an extra copy of chromosome 21, leading to what is known as trisomy 21. This genetic condition occurs during cell division, which results in an egg or sperm cell with an abnormal number of chromosomes. When this cell combines with a normal cell, the resulting embryo has three copies of chromosome 21 instead of the typical two. This extra genetic material disrupts normal development, causing the physical and intellectual characteristics associated with Down Syndrome.

The other choices describe factors that are not related to the genetic basis of Down Syndrome. For instance, folic acid deficiency during pregnancy is linked to neural tube defects and does not cause Down Syndrome. Similarly, Down Syndrome is not classified as a hereditary condition in the conventional sense, as most cases arise from spontaneous errors during cell division rather than being inherited from parents. Lastly, a lack of oxygen at birth can lead to various complications but is not a cause of Down Syndrome. The defining characteristic of Down Syndrome is specifically the presence of that additional chromosome.