When does trisomy 21 occur?


When does trisomy 21 occur?


Trisomy is a polysomy condition in which an extra chromosome is present along with the normal chromosomal set of two. Trisomy is an aneuploidy which results in a 2n+1 chromosome number in cells. In trisomy 21, or Down syndrome, chromosome 21 is represented by three, instead of two, copies.

Answer and Explanation: 1

Trisomy 21 occurs at the time of fertilization. However, it is the result of a meiotic error called non-disjunction (failure of homologous chromosomes or sister chromatids to properly separate) of chromosome 21, resulting in an extra chromosome in a given sex cell/gamete (egg or sperm).

A gamete should only have one copy of each chromosome, so that during fertilization, the offspring will receive 2 copies, one from each parent. If conception occurs between this atypical gamete with 2 copies of chromosome 21 and a normal gamete with only a single copy of chromosome 21, the offspring will have 3 copies of the chromosome, or Trisomy 21. Trisomy 21 is commonly known as Down syndrome. There can also be two special types of Trisomy 21: mosaic and translocation.

Learn more about this topic:

Aneuploidy: Definition & Disorders


Chapter 12 / Lesson 2

Learn the definition of aneuploidy. Understand the different conditions of aneuploidy. Learn about aneuploidy disorders and the diagnosis of aneuploidy.

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