Down Syndrome Report The chromosomal abnormality involved in most cases of Down syndrome is trisomy-21, or the presence of three copies of the 21st chromosome. As a result, the affected person has 47 chromosomes in all body cells instead of the normal 46, although how this causes the condition's symptoms is not yet known. Scientists assume that the reason for the abnormal chromosomal assortment is the fertilization of an ovum having 24 chromosomes by a sperm with a normal assortment of 23, but they have also found that the sperm can carry the extra chromosome as well. The abnormal ovum or sperm is derived from a germ cell in which the pair of 21st chromosomes holds together