Down syndrome is a chromosomal condition related to chromosome 21. It affects 1 in 800 to 1 in 1000 live born infants.
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.
These facts are from human Genome research institute
Down syndrome is not inherited but is a disease which when the cells are in reproductive state and it could cause a disease to the baby.