Down syndrome, the most common genetic condition in the united states, is also called trisomy 21. what does this mean? a. a person with down syndrome has two copies of chromosome 21 but they're fused together. during early embryo development, these chromosomes didn't segregate properly. b. a person with down syndrome has only one copy of chromosome 21 instead of two copies. either the sperm cell or the egg cell was lacking this chromosome. c. a person with down syndrome has three copies of chromosome 21 instead of two copies. an extra chromosome 21 was present in either the sperm or the egg cell. d. a person with down syndrome has three copies of chromosome 21 instead of two copies. two sperm cells simultaneously fertilized the egg cell.