What Is Down Syndrome

What is Down Syndrome?

Down syndrome is a genetic condition that causes delays in physical and intellectual development. It occurs in approximately one in every 800 live births. Individuals with Down syndrome have 47 chromosomes instead of the usual 46. It is the most frequently occurring chromosomal disorder. Down syndrome is not related to race, nationality, religion or socioeconomic status. The most important fact to know about individuals with Down syndrome is that they are more like others than they are different.

What is Down syndrome?

Down syndrome is a chromosomal condition that is associated with mental retardation, a characteristic facial appearance, and poor muscle tone (hypotonia) in infancy. People with Down syndrome are at an increased risk for heart defects, digestive problems such as gastroesophageal reflux or celiac disease, and hearing loss. Additionally, some people with Down syndrome have reduced activity of the thyroid gland (hypothyroidism). The thyroid gland is a butterfly-shaped organ in the lower neck that produces hormones.

What are the genetic changes related to Down syndrome?

Down syndrome is a chromosomal condition related to chromosome 21.

Most cases of Down syndrome result from trisomy 21, which means each cell in the body has three copies of chromosome 21 instead of the usual two copies. The extra genetic material disrupts the normal course of development, causing the characteristic features of Down syndrome. A small percentage of cases occur when only some of the body's cells have an extra copy of the chromosome. These cases are called mosaic Down syndrome.

Although uncommon, Down syndrome also can occur when part of chromosome 21 becomes attached (translocated) to another chromosome before or at conception. Affected people have two copies of chromosome 21, plus extra material from chromosome 21 attached to another chromosome. These cases are called translocation Down syndrome.

Can Down syndrome be inherited?

Most cases of Down syndrome are not inherited, but occur as random events during the formation of reproductive cells (eggs and sperm).