How Genetic Disorders Are Inherited

Genetic disorders are caused by the mutation of a gene. This genetic mutation can be passed from parents to a child at the time of conception. Whether the child will develop the genetic disorder depends largely on the pattern of inheritance.

There are two rules that predict the likelihood that a person will inherit a genetic disorder. It doesn't necessarily guarantee that this will happen, but it does increase the risk.

For example, Huntington's disease, breast cancer and autoimmune diseases are largely believed to be associated with genetics. However, the presence of a mutation doesn't always translate to the disease. On the other hand, some genetic mutations, such as those associated with hemophilia, will manifest characteristics of the disorder (albeit to different degrees of severity).

The determination of risk is based on the following:

• Whether one copy of the mutated gene (from one parent) or two copies (from both parents) are inherited
• Whether the mutation is on one of the sex chromosomes (X or Y) or one of 22 the other pairs of non-sex chromosomes (called autosomes)

Based on the combined characteristics, we can classify the disorder as being X-linked recessive, Y-linked, autosomal recessive, or autosomal dominant.

In X-linked recessive disorders, the mutated gene occurs on the X (female) chromosome. Males have one X and one Y chromosome, so a mutated gene on the X chromosome is enough to cause the disorder.

Females, by contrast, have two X chromosomes, so a mutated gene on one X chromosome usually has less effect on a female because the non-mutated copy on the other largely cancels out the effect.

However, a female with the genetic mutation on one X chromosome would be a carrier of that disorder. What this tells us is, that a from a statistical standpoint, 50 percent of her sons will inherit the mutation and develop the disorder, while 50 percent of her daughters will inherit the mutation and become a carrier.

Because only males have a Y chromosome, only males can be affected by and pass on Y-linked disorders. All sons of a man with a Y-linked disorder will inherit the condition from their father.

With autosomal recessive disorders, the person requires two copies of the mutated gene—one from each parent—to have the disorder.

A person with only one copy will be a carrier. Carriers will in no way be affected by the mutation or have any signs or symptoms of the disorder. They can, however, pass the mutation to their children.

If both parents carry the mutation for an autosomal recessive disorder, the odds of their children having the disorder are as follows:

• 25 percent risk of inheriting both mutations and having the disorder
• 50 percent risk of inheriting only one copy and becoming a carrier
• 25 percent risk of not inheriting the mutation at all

Moreover, each child will have the same chances of inheriting the mutated gene.

In autosomal dominant disorders, the person only needs one copy of the mutated gene to have or develop the condition. Males and females are equally likely to be affected. Moreover, children of a person with an autosomal dominant disorder have a 50 percent risk of inheriting the disorder, as well.