Your study of the biology behind reproduction has taught you that human children receive 23 chromosomes from their mother and 23 chromosomes from their father. What is in a chromosome that allows it to create discrete physical traits such as hair color, eye color, and body type in a brand new human being?

The breakthrough in understanding heredity happened when scientists discovered DNA, or deoxyribonucleic acid. Each chromosome in a living cell contains one strand or molecule of DNA. Scientists, like everyone else at the time, already understood that traits were passed down through generations in recognizable patterns. In the late 20th century scientists made the astonishing discovery that all the information an organism needs to express its parents' traits is located on the DNA found inside every cell.