Published by: Very Well Health
Trisomy is when three copies of a chromosome are present instead of two (all chromosomes normally come in pairs). While most parents-to-be are familiar with Down syndrome and will undergo prenatal screening to detect it, there are other, potentially more serious trisomies that may occur, including Edwards syndrome, Patau syndrome, and others. Some may cause few, if any, symptoms, while others can lead to severe defects that make life—or even the pregnancy—unsustainable.
A gene is essentially a packaged bundle of chromosomes that contain all of the coded information related to our physiological makeup and metabolic function. Each gene typically contains 46 chromosomes, 23 of which we inherit from our mothers and fathers, respectively.
Of these, 22 pairs are autosomes, which determine our unique biological and physiological features. The 23rd pair is sex chromosomes (known as X or Y), which designate whether we are biologically female or male.
In rare instances, a coding error may occur when a cell divides during fetal development. Instead of splitting cleanly into the two identical chromosomes, the newly divided chromosome will have extra genetic material. This can lead to either a full trisomy (in which a complete third chromosome is created) or a partial trisomy (in which only part of the chromosome is copied). From this point forward, the error will be repeated and repeated as the cell continues to divide.