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A chromosome contains a single, long piece of DNA with many different genes. Every human cell contains 23 pairs of chromosomes. There are 22 pairs of numbered chromosomes, called autosomes, and one pair of sex chromosomes, which can be XX or XY. Each pair contains two chromosomes, one from each parent, which means that children get half of their chromosomes from their mother and half from their father.

A gene is a part of DNA that carries the information needed to make a protein. People inherit one copy of each gene from their mother and one copy from their father. The genes that a person inherits from his or her parents can determine many things. For example, genes affect what a person will look like and whether the person might have certain diseases.

DNA is made up of two strands that wind around each other and looks like a twisting ladder. A DNA strand is made up of four different “bases” arranged in different orders. These bases are T (thymine), A (adenine), C (cytosine), and G (guanine). DNA is “read” by the order of the bases, that is by the order of the Ts, Cs, Gs, and As. The specific order, or sequence, of these bases determines the exact information carried in each gene (for example, instructions for making a specific protein). DNA has the same structure in every gene and in almost all living things.

A mutation is a change in a DNA sequence. DNA mutations in a gene can change what protein is made. Mutations present in the eggs and sperm (germline mutations) can be passed on from parent to child, while mutations that occur in body cells (somatic mutations) cannot be inherited.

A protein is made up of building blocks called amino acids. The main role of DNA is to act as the instructions for making proteins. It is actually proteins that make up most of the structures in our bodies and perform most of life’s functions. For example, proteins make up hair and skin. Proteins in our eyes change shape in response to light so we can see. Proteins in our bodies break down food. Proteins are made in cells and are the major parts of cells, which are the vital working units of all living things.

Environmental factors can include exposures related to where we live as well as behaviors such as smoking and exercise and cultural factors such as foods that we eat.