More than 24,000 children are born with hearing loss in the United States each year. Boys Town National Research Hospital is a national leader in the diagnosis and treatment of children with moderate to profound hearing loss.

Information on Hearing Loss - Genetics and Deafness - Glossary of Genetic Terms

Genes

Genes are the basic unit of inheritance. They provide the instructions for growth and development of a fertilized ovum into a baby and many continue to provide instructions for bodily functions throughout a person's lifetime. The genes are arranged in string-like structures which are called chromosomes.

Chromosomes

There are 46 chromosomes or 23 pairs of chromosomes. One chromosome in each pair comes from the individual's mother and the other one from the father. If the chromosomes are stained and examined under a microscope, one sees that they are of varying lengths and have light and dark bands. This is what makes it possible to identify the individual chromosomes. For standard identification, 22 pairs of chromosomes (autosomes) have been given numbers. The twenty-third pair is called the sex chromosomes because they have the genes that determine the individual's sex. Females have a matched pair of sex chromosomes called X chromosomes. Males have a mismatched pair with one X chromosome and one Y chromosome. The Y chromosome is smaller and does not have the same genes as the X.

Ovum and sperm cells (gametes) come from cells that have the 23 pairs of chromosomes. In order to become gametes the chromosomes in each original cell have to split up so that there is only one from each pair (a total of 23 instead of 46 chromosomes) in the mature gamete. Sometimes the chromosomes in one pair do not split up so that one gamete has two chromosomes from a pair and another gamete will have none from that pair. If either of those gametes is fertilized, there will be an extra copy or a missing copy of all of the genes carried on that particular chromosome. This is what causes the condition called Down Syndrome; there is an extra copy of the #21 chromosome. Sometimes Down Syndrome is called Trisomy 21 meaning three copies of the #21. A couple who has had a child with a trisomy has a 1% chance of having another child with a trisomy. Less frequently a chromosome abnormality results from loss of a piece or an extra piece of a chromosome or two chromosomes stuck together. The chance of recurrence in another child is dependant on the cause of the abnormality in each family.

Most genetic conditions are caused by a single gene or a single pair of genes. There are three main patterns of inheritance associated with these conditions.

Autosomal Dominant

An autosomal gene is on one of the autosomes or 22 pairs of chromosomes that are not sex chromosomes. Dominant means that the gene is expressed even when there is only one copy of the gene. A person with an autosomal dominant hearing loss has one gene for the loss. The other gene in that pair is probably a recessive gene that codes for normal hearing. That individual has a 50% chance of passing on the hearing loss gene to each child. People in the same family who don't have the hearing loss gene, can not pass it on to their children.

Autosomal Recessive

Autosomal recessive means that the gene is not expressed unless there are two copies of the gene. Usually the autosomal recessive hearing loss genes have been on both sides of the family for many generations but noone has had a hearing loss. Those people who have one copy of the gene and no hearing loss are called carriers. Their other gene in that particular pair provides the dominant message for normal hearing. When two carriers have a child, there is a 25% chance the child will receive both hearing loss genes and have a hearing loss. There is a 25% chance that the child will receive neither hearing loss gene and a 50% chance that the child will be a carrier like the parents.

X-linked Recessive

X-linked refers to a gene that is on the X chromosome; most X-linked hearing loss genes are recessive. A woman with an X-linked hearing loss gene will most likely have a dominant gene for normal hearing on her other X chromosome and be a carrier. If she has a child, there is a 50/50 chance of passing on the recessive gene. If a son receives the recessive gene, he will have the hearing loss because as a male he will have only one X chromosome. There will be no dominant gene for normal hearing on his Y chromosome. If a daughter receives the X chromosome she will most likely receive a dominant gene for normal hearing on the X chromosome from her father and be a carrier like her mother. A male with the X-linked recessive gene cannot pass it on to his son because he has to give his son a Y chromosome. However, all of his daughters will be carriers because he has only the one X chromosome with that gene to give to a daughter.

Date Originally Created: Fall of 1990

The information presented here first appeared in publications of the Boys Town National Research Register for Hereditary Hearing Loss, the National Institute on Deafness and Other Communication Disorders (NIDCD), Hereditary Hearing Impairment Resource Registry (HHIRR), or the Boys Town Research Registry for Hereditary Hearing Loss.

The Boys Town Research Registry for Hereditary Hearing Loss

The Boys Town Research Registry for Hereditary Hearing Loss (Registry) is designed to foster a partnership between families, clinicians and researchers in the area of hereditary hearing loss/deafness through three primary functions. First, the Registry disseminates information to professionals and families about clinical and research issues related to hereditary deafness/hearing loss. Second, the Registry collects information from individuals interested in supporting and participating in research projects. This information is used to support the third function of the Registry - matching families with collaborating research projects.

For more information, contact us at:

Research Registry for Hereditary Hearing Loss
555 N. 30th Street Omaha, NE 68131
800 320-1171 (V/TDD)
402 498-6331 (FAX)