What is Down Syndrome
Down Syndrome is a chromosomal disorder that is caused by an error in cell division called nondisjunction Trisomy 21. However, two other types of chromosomal abnormalities, Translocation and Mosaicism are also implicated in Down syndrome although to a much lesser extent. Each type of Down syndrome have an extra critical portion of chromosome 21 present in all or some of their cells. This additional genetic material alters the course of development and cause the characteristics associated with the syndrome.
Down Syndrome Biology 101:
The human body is made up of cells; all cells contain a center called the nucleus, in which genetic material known as genes is stored. This genetic material known as genes carries the codes responsible for all inherited characteristics. Genes are grouped along like a rod-like structure called chromosomes. Typically the nucleus of each cell contains 23 pairs of chromosomes, half of which are inherited from each parent giving us 46 total. In individuals with Down Syndrome the cells contain 47 chromosomes; the extra chromosome being the 21st. This excess genetic material add additional genes resulting in Down Syndrome. The extra 21st chromosome is detected by using a procedure called karyotype. A karyotype is a visual display of chromosomes grouped by size, number and shape. Chromosomes may be studied by examining blood or tissue cells. Individual chromosomes are identified, stained, and numbered from largest to smallest.
- Types of Down Syndrome -
Trisomy 21 is a faulty cell division that results in an embryo with three copies of chromosome 21 instead of the usual two. Prior to or during conception, a pair of the 21st chromosomes in either the sperm or egg fails to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. This error during the cell division is responsible for 95% of all people born with Down Syndrome. Why nondisjunction Trisomy 21 occurs is currently unknown. Heridity is not a factor in Trisomy 21.
Translocation accounts for about 4% of all cases of Down syndrome. In translocation, part of chromosome 21 breaks off during cell division and attaches to another chromosome, typically chromosome 14. While the total number of chromosomes in the cells remain 46, the presence of an extra part of chromosome 21 causes the characteristics of Down syndrome. In one third of cases of Down syndrome resultuing from translocation there is a heriditary compontent, accounting for about 1% of all cases of Down syndrome.
Mosaicism accounts for about 1% of all cases of Down syndrome. Research has indicated that individuals with mosaic Down syndrome may have fewer characteristics of Down syndrome than those with other types of Down syndrome. However, broad generalizations are not possible due to the wide range of abilities people with Down syndrome possess. Mosaicism occurs when nondisjunction of chromosome 21 takes place in one, but not all of the initial cell divisions after fertilization. When this occurs, there is a mixture of two types of cells, some containing the usual 46 chromosomes and others containing 47. Those cells with 47 chromosomes contain an extra chromosome 21. Heridity is not a factor mosaicism.
What are the chances of having a child with Down Syndrome:
One in every 691 babies in the the United States is born with Down syndrome, making Down syndrome the most common genetic condition. Roughly 400,000 Americans have Down syndrome and about 6,000 babies with Down syndrome are born in the United States each year. Down syndrome occurs in people of all races and economic status. Research has shown that older women have an increased chance of having a child born with Down Syndrome. However 80% of people born with Down Syndrome are born to women under 35 years of age. This occurs because younger women have higher fertility rates. A 35 year old woman has about a one in 350 chance of conceiving a child with Down syndrome, and this chance increases gradually to 1 in 100 by age 40. At age 45 the incidence becomes approximately 1 in 30. One in every 690 babies in the the United States is born with Down syndrome, making Down syndrome the most common genetic condition. Approximately 400,000 Americans have Down syndrome and about 6,000 babies with Down syndrome are born in the United States each year.