Aperson with down syndrome has an additional #21 chromosome in every somatic cell. this condition is also called trisomy 21. the cause of this syndrome a) is not known b) is due to crossing over of genes during prophase 1 of meiosis c) is through the union of a normal gamete with the haploid amount of chromosomes (n) and another gamete with an extra #21 (n + 1) d) is through a mutation during dna replication of mitosis in a somatic cell, and this mutation spreads to all other somatic cells in the body