Trisomian

Trisomian is a term used to describe an organism that possesses trisomy, a type of aneuploidy where an organism has three copies of a particular chromosome instead of the usual two. This condition arises from nondisjunction during meiosis, a process of cell division that produces gametes (sperm and egg cells). Nondisjunction occurs when homologous chromosomes fail to separate properly during meiosis I or when sister chromatids fail to separate during meiosis II. The resulting gametes will then have either an extra chromosome or a missing chromosome.

When a gamete with an extra chromosome fuses with a normal gamete, the resulting zygote will have three copies of that specific chromosome. This extra genetic material can lead to a variety of developmental abnormalities, the severity of which depends on the specific chromosome involved and the extent of genetic expression. In humans, trisomy is a significant cause of genetic disorders. The most well-known example is Trisomy 21, which causes Down syndrome. Other examples include Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome). While some trisomies are compatible with life, many are not, leading to spontaneous abortions or stillbirths. The genetic makeup of a trisomian organism is therefore characterized by an altered chromosomal number.