Understanding inherited genes

Genetic disease inheritance is complicated and falls into several different patterns; the four most common are:

• Autosomal recessive: A child must inherit the mutation from both parents to have the disease. A child whose parents both carry the gene has a 25 percent chance of having the disease, a 50 percent chance of being a carrier of the disease but not having it, and a 25 percent chance of not inheriting the gene at all. Cystic fibrosis and sickle cell anemia are examples of autosomal recessive diseases. Approximately 80 percent of children born with an autosomal recessive disorder have no evidence of the disease itself in their families. That’s because family members can carry a gene for autosomal recessive disease for generations . . . or forever, without the disease showing up, if no one marries a partner with the same recessive gene. Without two copies of the mutation, one from each parent, a child will not be born with the actual disease.
  
• Autosomal dominant: The child will have the disease even if only one parent passes on the mutation. Huntington’s chorea is an example of this disease. Huntington’s chorea can be passed on because its carriers usually survive long enough to have children and pass on the gene.
  
• X-linked dominant: An X-linked disease is a mutation of the X chromosome. Females are more often affected because they have two X chromosomes. Men can only pass an X chromosome to a daughter (girls have XX sex chromosomes, boys have XY, and mom always passes on one of the Xs) so men can’t pass X-linked disease to their sons. Fragile X Syndrome is an example of this disease.
• X-linked recessive: Both parents must pass on the gene for the child to be affected; one in four will have the disease, two in four will be carriers, and one in four will be free of the disease. Hemophilia is an example of an X-linked recessive disease. More males than females are affected by X-linked recessive disease, and males can’t pass them on the sons.