The 23rd pair are the sex chromosomes which determine whether we are male or female. Males have an X and a Y chromosome and females have two X chromosomes. Genes are the basic units of heredity, and line up on a chromosome like beads on a string. We each have around 80,000-100,000 genes that determine what we look like and how our bodies work. A gene is comprised of a series of 3-letter words and each word refers to a specific protein building block.
Some proteins are structural, in that they help to form tissue like the heart or bones.
Other types of proteins are critical in the thousands of chemical processes that occur in our cells to make our bodies function properly. All plants, animals and humans have genes that don't work as well as they should. They have been altered or changed by a process known as mutation. However, some mutations are harmful and cause disease. Spontaneous mutations occur in everyone, but usually enzymes repair them.