Genes are the blueprints for making the substances, called proteins, our bodies need to develop and work properly. Most genes come in pairs, one of which is inherited from the mother and the other from the father. A mutation is a change in a gene that causes it to be different from the copy inherited from either parent. Genes that are inherited from our biological parents are expressed in specific ways. One of the basic patterns of inheritance and expression of our genes is called autosomal recessive inheritance.
There are two types of chromosomes: the sex chromosomes (chromosome pair 23), which determine male or female genes, and all the other chromosomes (pairs 1 through 22). Autosomal inheritance of a gene means that the gene is located on one of the other chromosomes. This means that males and females are equally likely to inherit the gene copy. "Recessive" means that both copies of the gene must be inherited for a person to have the trait. One copy of the gene is inherited from the mother, and one from the father. A person who has only one recessive gene mutation is said to be a "carrier" for the trait or disease, but he or she does not have any of the recessive gene. Most people do not know they carry a recessive gene for a disease until they have a child with the disease. Once parents have had a child with a recessive disease, there is a 1out of 4, or 25% chance, with each subsequent pregnancy, for another child to be born with the same disorder. This means that there is a three out of four, or 75 percent chance, for another child to not have the disease.
The birth of a child with a recessive condition is often a total surprise to a family, since in most cases, there is no previous family history of the condition. Many autosomal recessive conditions occur this way. It is estimated that all people carry some recessive genes that could potentially cause genetic diseases or conditions. It is only when a person has a child with a partner that carries the same recessive gene copy, that there is a chance of having a child with a recessive disorder. Mutations in certain genes have occurred over time in different parts of the world. Virtually anyone can carry a mutation in any one of the recessive genes; however, there may be certain ethnic groups more likely to carry certain recessive gene mutations, because of where the mutation originated. For example, people of African descent are more likely to have sickle cell disease. Cystic fibrosis is more common in Caucasians.