Down's Syndrome, it is one of the most frequently occurring chromosomal abnormalities found in humans effecting people of all ages, races and economic levels. It is a chromosomal anomaly in cell development that results in a person being born with forty-seven chromosomes instead of the normal forty-six chromosomes. People with Down syndrome may have mild to severe learning disabilities and physical symptoms, which include a small skull, extra folds of skin under the eyes, and a protruding tongue.
Roughly one out of every one thousand children born making it the most common genetic disorder. Down syndrome affects over 350,000 people, in the United States alone.
Down syndrome has plagued the human race since the beginning of time. For centuries, people with Down syndrome have been depicted in art, literature and science as outcasts and rejects of society. It wasn't until the late 19th century, however, that John Langdon Down, an English physician, published an accurate description of a person with Down syndrome. It was this publication in 1866, which earned Down the recognition as the "father" of the syndrome. Although other people had previously recognized the characteristics of the syndrome, it was Down who described the condition as a distinct and separate entity. Throughout the 20th century, advances in medicine and science enabled researchers to investigate the characteristics of people with Down syndrome. In 1959, the French physician, Jerome Lejeune, identified Down syndrome as a chromosomal anomaly. Instead of the usual 46 chromosomes present in each cell, Lejeune observed 47 in the cells of individuals with Down syndrome. It was later determined that an extra partial or complete 21st chromosome results in the characteristics associated with Down syndrome.
Down syndrome, being a genetic disorder, begins to develop at the cellular level after conception. The human body is made of cells. All of these cells contain a center, called a nucleus, in which genetic code of each human being is stored. This genetic material, known as genes, carries the codes responsible for all our inherited characteristics from the mother and the father. Genes are grouped along rod-like structures called chromosomes. Normally, the nucleus of each cell contains 23 pairs of chromosomes, half of which are inherited from each parent. In Down syndrome, however, the cells usually contain not 46, but 47 chromosomes. The extra chromosome, being a number 21. This excess genetic material, in the form of additional genes along the 21st chromosome, results in Down syndrome. Because 95 percent of all cases of Down syndrome occur because there are three copies of the 21st chromosome, it is often referred to as "trisomy 21." Chromosomes may be studied by examining blood or tissue cells. Individual chromosomes are identified, stained and numbered from largest to smallest. The visual display of the chromosomes is known as a karyotype.
Down syndrome is usually caused by an error in cell division called non-disjunction. However, two other minor types of chromosomal abnormalities, mosaicism and translocation, are also causes of Down syndrome. Regardless of the type of Down syndrome that a person may have, all people with Down syndrome have an extra, critical portion of the number 21chromosome present in all, or some, of their cells. This additional genetic material alters the course of development and causes the characteristics associated with the syndrome.
Non-disjunction is a faulty cell division that results in an embryo with three number 21 chromosomes instead of two. Prior to, or at, conception, a pair of number 21 chromosomes, in either the sperm or the egg, fail to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. This faulty cell division is responsible for 95 percent of all cases of Down syndrome. Why nondisjunction occurs is currently unknown, although it does seem to be related to advancing maternal age. What is ironic is that 80 percent of children born with Down syndrome are born to women under 35 years of age. This is because younger women have higher fertility rates. What is even more amazing is that it does not contradict the fact that the chance of having a child with Down syndrome increases with the age of the mother. Although nondisjunction can be from the father, this occurs less frequently the case. Because this error in cell division is