Trisomy+21-+Down+Syndrome

= Wiki Post #1-Genetics: October 11th, 2013` =


 * Down syndrome **, the most common chromosome abnormality in humans, is a genetic disorder most commonly caused by the presence of all or part of a third copy of chromosome 21. ** Trisomy 21 ** accounts for 95% of Down syndrome cases. Down syndrome is named after the British physician // John Langdon Down // , who first described it in 1866. Down syndrome was first identified as a chromosome 21 trisomy by // Dr. Jérôme Lejeune // in 1959. It is generally not hereditary.

//A male with the presence of a third copy of chromosome 21//


 * What is Trisomy 21?**

Trisomy 21 occurs when there is a presence of a 3rd chromosome 21. Instead of having 46 chromosomes, those with Trisomy 21 have 47. This extra chromosome comes from an uneven separation of chromosomes during Meiosis. This error in separation is called **nondisjunction,** which can occur in both Meiosis I and Meiosis II.

//Nondisjunction during both Meiosis I and Meiosis II//

Rather than ending up with regular gametes with **n** chromosomes, nondisjunction can lead to gametes with an abnormal amount, like **n-1 (monosomy)** and/or **n+1 (trisomy).** When these abnormal gametes merge with other gametes with **n** chromosomes, the end product with also have an abnormal amount of chromosomes. In terms of Trisomy 21, a **n+1** gamete combines with a haploid sperm cell and causes trisomy.

media type="custom" key="24020066" //How nondisjunction occurs during Meiosis I and Meiosis II//


 * What are the symptoms of Trisomy 21?**

Individuals with Trisomy 21 do share some faint similarities in their physical features, like upward slanting eyes and a rounder face. However, like all children, children with Trisomy 21 resemble their parents for the most part.

//Common physical features for Trisomy 21//

Those with Trisomy 21 have intellectual disabilities that can range from mild to moderate, but can also be severe. Most children with it will be able to walk and talk, but it takes longer for them to develop these skills. The developmental process depends on the person and their environment. Trisomy 21 causes individuals to have a greater risk of having health problems, such as hearing and eye disorders or heart and gastrointestinal malformations. The life expectancy for those with Down syndrome has increased over the past few decades, and person with it in good health will on average live to age 55 and beyond.


 * Who can get Trisomy 21?**

Trisomy 21 is believed to not be hereditary, and any woman is able to give birth to a child with this syndrome. It affects roughly **1 in 770** babies at birth. The probability of having a baby with Trisomy 21 increases with the age of the mother, therefore younger mothers have lower risk of having a baby with it.

//The age of the mother is a proven factor of Trisomy 21//


 * Where can I get further information on Trisomy 21?**

Common questions about Trisomy 21 and Down syndrome answered: []

The story of Trisomy 21 and Down syndrome: []

The foundation directed towards Down syndrome research: []

The Canadian Down Syndrome Society official website: []

[] [] [] []
 * Sources Cited: **