Chances of Inheriting a
Recessive Disorder

Genetic disorders can be caused by a change(s) in a gene. Every individual has two copies of the same gene. Genetic disorders are inherited in different ways.

Recessive means:

• a person must inherit a change in the same gene from each parent in order to have the disorder
• a person with one changed gene does not have the disorder, but can pass either the changed or the unchanged gene on to his or her child

An individual with the disorder usually:

• has inherited a change in the same gene from each parent

An individual who has one changed gene is called a carrier. When two carriers of the same changed gene have a child together, with each birth there is a:

• 1 in 4 chance of having a child with the disorder
• 2 in 4 chance of having a child who is a carrier
• 1 in 4 chance of having a child who neither has the disorder nor is a carrier

Genetics of Enlarged Vestibular Aqueducts (EVA)

Genes are the means by which we inherit traits and disorders from our parents. Genes provide instructions to our cells to determine how we grow and develop, both in the womb and after we are born.

We have a pair of each gene for every trait we inherit. A child inherits one copy of most genes from the mother and another copy from the father. This happens when the egg and sperm come together at conception. Research from the Human Genome Project suggests that humans have between 20,000 and 25,000 pairs of genes in every cell of their bodies!

Mutations

Most genes are the same in all people, but a small number of genes (fewer than 1 percent of the total) are slightly different among people. These changes in a gene are often called mutations. Many of these mutations cause us no problems at all. For example, they may explain why you have blue eyes but your brother has brown—or why your best friend in high school could run very fast, while you were good at math.

Some changes in genes, though, can cause medical problems and disorders. Changes in a gene can result in a person's blood not clotting properly or cause weak, brittle bones. Changes in more than 60 different identified genes can cause hearing loss.

Inheritance

Most genetic disorders are inherited in an autosomal manner. Autosomal means that men and women are equally likely to have the disorder and equally likely to pass it on to a child of either sex. Disorders can also be inherited in a sex-linked manner. For most sex-linked traits, women rarely have the trait, and they pass it on only to their sons.

Sometimes a mutation in only one copy of a gene can cause a medical disorder. If this is the case, the trait is said to be inherited in a dominant manner. A recessive disorder, on the other hand, arises only when the mutation is present in both copies of the gene. Hearing loss can be inherited in a dominant or recessive manner, depending on the gene that is causing the hearing loss.

A person with one change in a gene that is responsible for an autosomal recessive disorder typically has no signs or symptoms of the disorder. These individuals are often referred to as carriers, because they "carry" a mutation but are not affected by it. Usually when an individual has a recessively inherited disorder, both parents are carriers of a gene mutation for the disorder.

EVA

Approximately one-third of individuals with EVA and hearing loss who have participated in an NIDCD study were diagnosed with Pendred syndrome. Pendred syndrome is inherited in a traditional autosomal recessive manner as described above. It is the result of two mutations in the SLC26A4 gene.

It is not clear what causes EVA in the other two-thirds of participants who do not have Pendred syndrome. Half of these individuals had one mutation in SLC26A4, and half had no mutations in SLC26A4. One research goal of the study is to identify other genes that may be responsible for EVA and hearing loss.

There is not yet adequate data to determine the likelihood that parents who have one child with EVA and hearing loss not due to Pendred syndrome will have another child with EVA and hearing loss. Nor do we know yet what the likelihood is that an individual with EVA and hearing loss but not Pendred syndrome will have a child with EVA and hearing loss.