How Autosomal Recessive Diseases Are Inherited
Canavan disease is one of many autosomal recessive conditions - others include cystic fibrosis, sickle cell disease, and Tay-Sachs disease - for which two copies of an abnormal gene must be present for the condition itself to be present. Most people have two fully-functioning genes. A carrier - who has one mutated gene and one functioning gene - does not show any impact of the disease because one functioning gene is still sufficient to produce the necessary enzyme, but can pass the mutation along to his or her children. A child who inherits two mutated genes, one from each parent, is affected by the disease.
Two carriers who have a child together have a one-in-four chance with each pregnancy of a child in which both genes carry the mutation and the enzyme is not produced, leading to the disease. There is a two-in-four chance of passing along just one mutated gene (carrier status) and a one-in-four chance that the child will have two fully functioning genes and be neither a carrier nor be affected by the disease.
