Our specific DNA sequence, which determines eye color and height, will play a role in our susceptibility to disease. This following discussion is included not so much because it is necessary to understand, but because it was one of the seminal scientific discoveries of the 20 th century. Additionally, the Watson-Crick model determined that as a result of chemical properties, adenine would only form a base pair with thymine, and guanine would only pair with cytosine. All species use this same four-letter code and human beings have a little more than 3 billion pairs of nucleotides in each cell. DNA is organized into discrete units termed genes.
Genes are the basic physical and functional units of heredity.
Half of the information encoded in our genes comes from our mother and the other half from our father. Generally speaking, a gene is a sequence of DNA that determines when, when , where, and how much of a given protein is produced in a living cell. The DNA code of a gene is very precise. As will be illustrated later, even a single change in the DNA sequence of a gene can have dramatic consequences. A variation in the DNA sequence of one individual compared to another is termed a polymorphism.