down syndrome
One common example of an extra-chromosome disorder is Down syndrome Figure below . Children with Down syndrome are mentally disabled and also have physical deformities. Down syndrome occurs when a baby receives an extra chromosome 21 from one of his or her parents. Usually, a child will receive one chromosome 21 from the mother and one chromosome 21 from the father. In an individual with Down syndrome, however, there are three copies of chromosome 21 Figure below . Therefore, Down syndrome is also known as Trisomy 21. These people have 47 total chromosomes.