Genes are the fundamental units of heredity determining our physical and biochemical characteristics. They are composed of DNA (deoxyribonucleic acid) and they are "packaged" into a series of chromosomes. Each of us has 23 pairs of chromosomes in the cells of our body; one member of each pair (and its genes) comes from our father and the other from our mother. 22 of the chromosome pairs are known as autosomal chromosomes and the 23rd pair of chromosomes are the sex chromosomes (X,Y).

The Jewish genetic diseases described here are autosomal recessive disorders and they result from altered genes (mutations) that are located on autosomal chromosomes. For these diseases to occur, an abnormal (mutated) gene leading to the disorder must be inherited from each parent. Carriers for an autosomal recessive disorders are individuals who have in their genetic composition one normal gene and one mutated gene for that specific disorder. Carriers are themselves healthy individuals and have no symptoms or signs of the disease for which they are carriers. The main concern for carrier individuals is that they are at risk of passing mutated genes to their children:

If both parents are carriers for the same disorder, there is a 50% likelihood with each pregnancy that the child will be a carrier (like each parent) and a 25% likelihood with each pregnancy that the child will be affected with the disorder. There is a 25% likelihood with each pregnancy that the child will be neither affected with the disorder nor a carrier for the disorder.