Down syndrome is a common chromosomal disorder in humans caused by the presence of an extra copy of chromosome 21. It is therefore also known as trisomy 21. In Human Genetics, it is an important example of a numerical chromosomal abnormality or aneuploidy. The condition affects physical development, intellectual development, and certain physiological functions. It occurs in individuals of all populations and both sexes. Normally, human body cells contain 46 chromosomes arranged in 23 pairs. In Down syndrome, there are usually 47 chromosomes because chromosome 21 is present in three copies instead of two. The most common form is caused by meiotic nondisjunction, in which chromosome 21 fails to separate properly during the formation of an egg or sperm. As a result, a gamete containing two copies of chromosome 21 may fuse with a normal gamete containing one copy, producing a zygote with three copies of chromosome 21. There are three major genetic forms of Down syndrome. The first and most common is trisomy 21, which accounts for about 95% of cases. In this form, all or nearly all body cells contain three copies of chromosome