Hemophilia results from a defect in the genetic material of the body. This genetic material is called deoxyribonucleic acid, or DNA. Any part of the DNA that controls an inherited trait is called a gene. Examples of inherited traits are eye color, hair color and blood type.

The most common forms of hemophilia result from defects in the genes that control the production of clotting factors VIII or IX.

Genes are present within the cell in packages called chromosomes. Most of the body's cells contain a complete copy of chromosomes and their genes. Among the chromosomes people inherit from their parents are two sex chromosomes, labeled X and Y. All males receive the X chromosome from their mother and the Y chromosome from their father. Females have two X chromosomes, one inherited from each parent.