Genetics Overview

Single Gene Disorders

When a certain gene is known to cause a disease, we refer to it as a single gene disorder or a Mendelian disorder. For example, you may have heard of cystic fibrosis, sickle cell disease, Fragile X syndrome, muscular dystrophy, or Huntington disease. These are all examples of single gene disorders. As a rule, single gene disorders are not very common. For example, only one in 2,500 people are born with cystic fibrosis. There are a number of inheritance patterns of single gene disorders that are predictable when you know what they are.

Autosomal dominant means that a person only needs one copy of the changed gene (genetic difference) in order to have the disorder. Usually, the changed gene is inherited from a parent who also has the disorder and every generation in the family may have members with the disorder. There are some instances in which a person has the gene that causes the disorder and does not show symptoms of the disorder, but can still pass the gene to his or her children. A person who carriers a gene for an autosomal dominant disorder has a 50% chance of passing the gene to each child.

Autosomal recessive means that it is necessary to have two copies of the changed gene to have the disorder. Each parent contributes one changed copy of the gene to the child who has the disorder. The parents are called carriers of the disorder because they have one normal copy of the gene and one changed copy of the gene, but they do not show symptoms of the disorder. When both parents are carriers of the changed gene, each of their children has a 25% chance of having the disorder, a 50% chance of being a carrier of the disorder (like their parents), and a 25% chance of neither being a carrier nor having the disorder. These risks are the same for each pregnancy. When there is more than one person in a family who has the disease, these people are often in the same generation.

X-linked dominant inheritance follows a pattern similar to autosomal dominant inheritance except that more females are affected than males. However, X-linked dominant disorders are very rare.

X-linked recessive disorders are usually only seen in males and they are much more common than X-linked dominant disorders. People with an X-linked recessive disorder do not have any normal copies of the gene. Males only have one X chromosome, so if a male inherits a changed gene on his X chromosome (which is always inherited from his mother), then he does not have another copy of the working gene to compensate. Females with one copy of a changed gene on one X chromosome are called carriers of X-linked recessive disorder. It is rare for a female to have the changed gene on both her X chromosomes. In most cases, females who are carriers do not show symptoms because the working copy of the gene compensates for the non-working copy of the gene. Carrier females have a 25% of having a son with the disorder, a 25% chance of having a son without the disorder, a 25% chance of having a carrier daughter and a 25% chance of having a daughter who is not a carrier. Males with an X-linked recessive disorder cannot pass the disorder to their sons, but 100% of their daughters will be carriers.