BY:Nithya Thiru and Simran Sethi
People who have Down syndrome have learning difficulties, mental retardation, a characteristic facial appearance, and poor muscle tone (hypotonia) in infancy.
Individuals with Down syndrome also have an increased risk for having heart defects, digestive problems such as gastroesophageal reflux or celiac disease, and hearing loss. Some people who have Down syndrome have low activity of the thyroid gland (hypothyroidism) - an organ in the lower neck that produces hormones.
Down syndrome can be diagnosed in infancy based on the characteristic clinical findings. When Down syndrome is suspected in a person, a genetic test called a chromosome analysis is performed on a blood or skin sample to look for an extra chromosome 21 (trisomy 21). Trisomy 21 means that each cell in the body has three copies of chromosome 21 instead of the usual two copies.
Having an extra number 21 chromosome interrupts the normal course of development, causing the characteristic clinical features of Down syndrome. Some people who have Down syndrome have an extra number 21 chromosome in only some of their body's cells. This type of Down syndrome is called mosaic Down syndrome.
A small number of individuals have Down syndrome because part of chromosome 21 becomes attached (translocated) to another chromosome before or at the time of conception. These individuals have two copies of chromosome 21, and additional material from chromosome 21 that is attached to another chromosome. The chromosomes of parents of a child with Down Syndrome caused by a translocation are studied to see whether the translocation was inherited.
Is Down syndrome inherited?
Most cases of Down syndrome are not inherited, but occur as random events during the formation of reproductive cells (eggs and sperm). An error in cell division called nondisjunction results in reproductive cells with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of chromosome 21. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra chromosome 21 in each of the body's cells.
Mosaic Down syndrome is also not inherited. It occurs as a random error during cell division early in fetal development. As a result, some of the body's cells have the usual two copies of chromosome 21, and other cells have three copies of the chromosome. Translocation Down syndrome can be inherited. An unaffected person can carry a rearrangement of genetic material between chromosome 21 and another chromosome. This rearrangement is called a balanced translocation because there is no extra material from chromosome 21. Although they do not have signs of Down syndrome, people who carry this type of balanced translocation are at an increased risk of having children with the condition.