Down syndrome is a genetic disorder caused by an extra copy of chromosome 21. Typically, a baby is born with 46 chromosomes, but babies with Down syndrome have an extra copy of one of these chromosomes, chromosome 21. This extra genetic material is responsible for the characteristic features and developmental problems of Down syndrome. The extra chromosome 21 leads to the physical features and developmental challenges that can occur among people with Down syndrome. Therefore, chromosome 21 is the chromosome affected in Down syndrome.