... acrocentric chromosomes (chromosomes 13, 14, 15, 21, and 22) join together, generating a fusion chromosome that contains virtually all of the genetic material of the original two chromosomes. ... often involving chromosomes 14 and 21. In this instance, the affected individual has 46 chromosomes, including one structurally normal chromosome 14, two structurally normal chromosomes 21, ... normal chromosomes 21, and one fusion 14/21 chromosome. This effect leads to a normal diploid dosage for chromosome 14 and to a triplication of chromosome 21, thus resulting in Down syndrome....