By: Landry LeeAnn Mikulec 2nd Period AP Biology Down Syndrome By: Landry LeeAnn Mikulec 2nd Period AP Biology

Chromosomal Connection Typically, the nucleus of each cell contains 23 pairs of chromosomes, half of which are inherited from each parent. Down syndrome occurs when an individual has a full or partial extra copy of chromosome 21. This additional genetic material alters the course of development and causes the characteristics associated with Down syndrome. One in every 691 babies in the United States is born with Down syndrome, making Down syndrome the most common genetic condition.

Discovery & History of Down Syndrome John Langdon Down is known as the “Father of Down Syndrome”. Down described the syndrome as a distinct and unique entity. This syndrome was discovered and published in 1866. Down syndrome is a result of a phenomenon called “nondisjunction.” Trisomy 21 (nondisjunction)

Genetic Inheritance of Down Syndrome Most cases of down syndrome are not inherited. When the condition is caused by trisomy 21, the chromosomal abnormality occurs as a random event during the formation of reproductive cells in a parent. Trisomy 21 refers to three copies of chromosome 21 present instead of the normal two. People with translocation Down syndrome can inherit the condition from an unaffected parent.

Translocation Down Syndrome This form of down syndrome happens when the parent carries a rearrangement of genetic material between chromosome 21 and another chromosome. This rearrangement is called balanced translocation, no genetic material is gained or lost within this translocation. However, when this translocation is passed on to generations it can become unbalanced causing Down Syndrome. There are no distinct cognitive or medical differences between people with translocation trisomy 21 and those with complete trisomy 21.

Mosaic Down Syndrome With mosaic trisomy 21, the error or misdivision occurs after fertilization at some point during early cell division. People with mosaic Down syndrome have two cell lines -- one with the normal number of chromosomes, and one with an extra number 21. Individuals with mosaic Down syndrome may or may not have milder disabilities and less obvious features of Down syndrome . Levels of mosaicism vary between individuals and within the cells of the individuals themselves, the effect of mosaicism is wide and variable.

Can Down syndrome be treated? Several clinical trials of drugs to improve memory and learning in adults with Down syndrome are now under way. If the trials succeed, the new drugs could help people with Down syndrome live more independently. Down syndrome has long been considered untreatable, but these new advances in neuroscience and genetics can change the treatment options of these three types of Down syndrome.