Down syndrome is a condition in which the person has an extra chromosome. Chromosomes are small "packages" of genes in the body. Babies with Down syndrome have an extra copy of one of these chromosomes: chromosome 21.
They determine how the baby's body is formed during pregnancy and how it works while it develops in the womb and after birth. Usually babies are born with 46 chromosomes. Babies with Down syndrome have an extra copy of one of these chromosomes: chromosome 21. The medical term for having an extra copy of a chromosome is ‘trisomy’. Down syndrome is also called trisomy 21. This extra copy changes the way the baby's body and brain develop, which can cause both mental and physical problems.Edit best answer History