Sometimes when meiosis (the division of reproductive cells) occurs, chromosomes may be lost, left behind, or too many may be passed on, resulting in the birth of a child with a genetic or congenital abnormality. or illness. One of the most common genetic disorders is Down syndrome. This disorder takes its name from Dr. Langdon Down, who was the first to formally describe it, in 1866. Down syndrome is a disease characterized by abnormal physical and mental development caused by a genetic abnormality. This genetic defect is caused by an extra chromosome. People born with Down syndrome have 47 chromosomes, compared to the normal 46, or 23 chromosomes inherited from each parent. This extra chromosome more often comes from the mother's egg than from the father's sperm. Chromosomes are large, individual DNA molecules located in the nucleus of a cell. They contain the genes as well as the structures that hold DNA and help it express heredity by orchestrating cellular operations. Down syndrome is one of the most common chromosomal abnormalities, occurring in approximately 1 in 800 births. However, the frequency of the condition varies greatly depending on the age of the mother. For women in their 20s, the rate is 1 in 2,000, but it increases sharply to 1 in 100 when a woman turns 40. Down syndrome is usually recognized shortly after birth due to characteristic facial features such as a flattened nose, an upward tilt of the nose. eyes, a large tongue which often protrudes from a small...