Autosomal recessive inheritance: Pattern of inheritance in which disease results only when an individual inherits two gene mutations for a particular disease. If both members of a couple are carriers of the same disease gene, there is a one in four or 25% chance in each pregnancy for a child to be affected.

Carrier: An individual who has one copy of an altered gene and one working copy of the same gene. Carriers are healthy individuals. Usually, the only consequence of being a carrier of a recessive genetic trait is the possibility of transmitting the particular genetic trait to a child.

Chromosome: A structure found in the nucleus of a cell that contains genetic information. Human cells contain 46 chromosomes, which are in pairs. Each chromosome contains thousands of individual genes, which determine an individual's characteristics.

DNA (deoxyribonucleic acid): The molecule that encodes the genes responsible for the structure and function of living organisms and allows the transmission of genetic information from generation to generation.

Enzyme: A protein that acts as a catalyst to cause chemical changes in other substances. Enzymes are often required for the normal metabolism, or breakdown, of substances in the body.

Gene: The fundamental unit of heredity. A gene is composed of a sequence of DNA required to produce a functional protein. The information in the genes is passed from parent to child.

Genetic Counseling: The communication process that helps individuals, couples or families to understand genetic information such as occurrence, or the risk of occurrence, of a genetic disorder in a family. Genetic counselors help individuals make informed decisions about family planning.

Mutation: A change in the sequence of DNA. When mutations occur in genes and disrupt the production of a functional protein, this may lead to genetic disease.