Genes are the smallest units of heredity. The information from all the genes, taken together, makes up the blueprint or plan for the human body and its functions. A gene is a short segment of DNA which is interpreted by the body as a plan or template for building a specific protein. Genes reside within long strands of DNA which in turn make up the chromosomes.

Some diseases, such as sickle cell anemia, can be caused by a change in a single gene (one out of the millions of genes which make up the plan for the entire human body). Genes are arranged in order along the DNA strand within the chromosome (similar to beads on a string). Matching genes from each parent exist on matching chromosomes and matching positions along the DNA within the chromosome. These genes are paired, one from the mother and one from the father. Genes are described as DOMINANT or RECESSIVE.

DOMINANT means that one gene in the gene pair is able to control the trait which that gene pair codes for. RECESSIVE means that both genes in the gene pair are necessary to control the trait.