A trisomy is a type of polysomy in which there are three instances of a particular chromosome, instead of the normal two.^[1] A trisomy is a type of aneuploidy (an abnormal number of chromosomes).

Most organisms that reproduce sexually have pairs of chromosomes in each cell, with one chromosome inherited from each parent. In such organisms, a process called meiosis creates cells called gametes (eggs or sperm) that have only one set of chromosomes. The number of chromosomes is different for different species. Humans have 46 chromosomes (i.e. 23 pairs of chromosomes). Human gametes have only 23 chromosomes.

If the chromosome pairs fail to separate properly during cell division, the egg or sperm may end up with a second copy of one of the chromosomes. (See non-disjunction.) If such a gamete results in fertilization and an embryo, the resulting embryo may also have an entire copy of the extra chromosome.

Terminology

The number of chromosomes in the cell where trisomy occurs is represented as, for example, 2n+1 if one chromosome shows trisomy, 2n+1+1 if two show trisomy, etc.[2]

“Full trisomy”, also called “primary trisomy”,[2] means that an entire extra chromosome has been copied.
“Partial trisomy” means that there is an extra copy of part of a chromosome.
“Secondary trisomy” – the extra chromosome has quadruplicated arms (the arms are identical; it is an “isochromosome”).[2] “Tertiary trisomy” – the extra chromosome is made up of copies of arms from two other chromosomes.[2]

Trisomies are sometimes characterised as “autosomal trisomies” (trisomies of the non-sex chromosomes) and “sex-chromosome trisomies.” Autosomal trisomies are described by referencing the specific chromosome that has an extra copy.[citation needed] Thus, for example, the presence of an extra chromosome 21, which is found in Down syndrome, is called trisomy 21.

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