Polysomia

Polysomia refers to a condition where an individual has more than the usual number of chromosomes. In humans, the normal complement is 46 chromosomes, arranged in 23 pairs. Polysomia occurs when there are extra chromosomes beyond this standard number. The most common type of polysomia in humans is trisomy, where there is an extra copy of one specific chromosome, resulting in a total of 47 chromosomes.

Trisomy can affect any chromosome. For example, Trisomy 21 leads to Down syndrome, Trisomy 18 causes Edwards syndrome, and Trisomy 13 results in Patau syndrome. These conditions are often associated with a range of physical and intellectual disabilities. While trisomy is the most frequent form of polysomia, other types exist, such as tetrasomy (four copies of a chromosome) and pentasomy (five copies of a chromosome), though these are much rarer and often more severe.

Polysomia typically arises due to errors during cell division, specifically nondisjunction, where chromosomes fail to separate properly during meiosis (the process of forming eggs and sperm) or mitosis (cell division in the rest of the body). This can lead to gametes (sperm or egg cells) with an abnormal number of chromosomes, which then result in an embryo with a polysomic condition if fertilization occurs. The exact causes of nondisjunction are not always understood but can be influenced by factors like maternal age. Diagnosing polysomia is often done through prenatal testing such as amniocentesis or chorionic villus sampling, or after birth through karyotyping.