noun, plural: trisomies
Aneuploidy is a chromosomal variation due to a loss or a gain of one or more chromosomes resulting in the deviation from the normal or the usual number of chromosomes. The different conditions of aneuploidy are nullisomy (2N-2), monosomy (2N-1), and polysomy (e.g. trisomy and tetrasomy). Polysomy is a condition where there is more than one copy of the chromosome relative to the normal. Trisomy (2N+1) and tetrasomy (2N+2) are examples of polysomy.
Trisomy occurs because of nondisjunction, i.e. when one of the chromosome pairs fails to separate during cell division, particularly, meiosis. Meiosis is a form of cell division to produce gametes. When a gamete containing an extra chromosome becomes a part of fertilization producing an embryo, the embryo would be trisomic. An extra copy of the entire chromosome is referred to as full trisomy. If there is an extra copy of a part of the chromosome it is a partial trisomy. If the extra copy is a non-sex chromosome, it is referred to as autosomal trisomy. If it is a sex chromosome, it is called a sex-chromosome trisomy.
An example of trisomy in humans is trisomy 21 syndrome (or Down syndrome) where there is an extra chromosome 21.
- trisomic (adjective, of, relating to, or characterized by, trisomy; noun, a cell or an organism in trisomy and a chromosomal composition designated as 2N+1)