Trisomy is the most common type of aneuploidy, conditions caused by an abnormal number of chromosomes. Specifically, trisomy is a classification of conditions that feature three copies of a chromosome rather than the standard two. Most humans are born with two pairs of 23 chromosomes, for a total of 46. Occasionally, an issue occurs while the body is producing gametes such as sperm and eggs that causes the gametes to develop trisomy. This can have a variety of effects depending on the chromosomes of which there are extras.
For the human body to create gametes, it uses the process of meiosis. The goal of meiosis is to create daughter cells that have exactly half as many chromosomes as the starting cell. Essentially, meiosis is a division process that takes diploid cells with two sets of chromosomes and divides them into haploid cells that have a single set of chromosomes. When an egg and sperm join together in fertilization, the haploids from each cell merge and form a complete diploid. Trisomy appears after a failed cell division.
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