Associated disorders:

Acute myeloid leukemia (CBF-AML) responds to a genetic rearrangement (translocation) that involves chromosome 21. This reorganization occurs in approximately 7 percent of cases of acute myeloid leukemia in adults. The translocation, written as t (8; 21), fuses part of the RUNX1 gene on chromosome 21 with part of the RUNX1T1 gene (also known as ETO) on chromosome 8. This mutation is somatic, so it is not inherited. This change in gene activity blocks the maturation (differentiation) of blood cells and leads to the production of abnormal immature white blood cells called myeloid blasts. While t (8; 21) is important for the development of leukemia, one or more additional genetic changes are usually needed for myeloid blasts to develop in leukemia cancer cells.

Down's syndrome is a chromosomal condition that is associated with intellectual disability, a characteristic facial appearance and a weak muscle tone (hypotonia) in childhood. This condition is most often caused by Trisomy 21. Trisomy 21 means that each cell in the body has three copies of chromosome 21 instead of the usual two copies. Less frequently, Down syndrome occurs when part of chromosome 21 joins (translocates) to another chromosome during the formation of reproductive cells (ovules and sperm cells) or very early in fetal development. Affected people have two copies of chromosome 21 plus additional material from chromosome 21 attached to another chromosome, which results in three copies of the genetic material of chromosome 21. It is said that people affected with this genetic change have Down translocation syndrome .
Down syndrome is associated with intellectual disability, a characteristic facial appearance and a weak muscle tone (hypotonia) in childhood. All affected individuals experience cognitive delays, intellectual disability is usually mild to moderate. In most people there are heart defects and, to a lesser extent, digestive problems such as gastroesophageal reflux. Celiac disease or gluten intolerance also usually occurs. They also tend to suffer from hypothyroidism and hearing and vision problems. A small percentage will develop some type of blood cancer (Leukemia).
Late development and speech and behavior problems that range from rebellious to autistic spectrum disorders, may occur. The degeneration of the mental faculties usually accelerates in these people with greater propensity to suffer Alzheimer's from the age of fifty.

Several types of cancer are due to translocations of genetic material between chromosome 21 and other chromosomes. For example, Acute lymphoblastic leukemia (a type of blood cancer most frequently diagnosed in childhood) has been associated with a translocation between chromosomes 12 and 21.

Other changes in the amount or structure of chromosome 21, such as Monosomy 21 or ring structure have a variety of effects on health and development. Chromosome 21 abnormalities can cause intellectual disability, delayed development and characteristic facial features. In some cases, the signs and symptoms are similar to those of Down syndrome.