St. Luke’s Maternal Fetal Medicine

Chromosomes

Chromosomes are tiny threadlike structures within the body cells, which carry the genetic instructions for an individual’s unique traits and characteristics (i.e. hair color, eye color). In a normal human cell there are 46 total chromosomes. These come in pairs, with one of each pair coming from a person’s mother, the other from their father. The first 22 pairs are identical for males and females, and are called the “autosomes”. The 23rd pair determines the sex of an individual. Females have two X chromosomes; males have one X and one Y.