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 -
X-Linked Dominant Inheritance
Genes are the instructions found in each cell of the body that determine how the body is to develop and function. Genes are strung together, one after the other, and packaged into individual chromosomes, which are transmitted from parent to child in a very regular fashion. All chromosomes come in pairs; in humans, there are 23 pairs in all. One chromosome from each pair is inherited from the mother, and one from the father.
A gene that codes for a particular function may come in several different forms; that is, a gene that controls pigment in hair may code for blond hair or black hair. The two genes in a pair may not agree. If the code for one gene is expressed no matter what the other gene says, the first gene is said to be dominant. If both of the genes have to be the same in order for that code to be expressed, those genes are said to be recessive. The commonly used example is that a person with one gene for black hair would have black hair, whether or not the other gene was for blond hair. Blond hair would show up only if both of the genes in the pair were for blond hair.
This is an oversimplification of the genetics of hair color, but it does illustrate the basic definition of dominant and recessive.
For twenty-two of the pairs of chromosomes, the chromosomes within the pair look alike. These are called the autosomes. The twenty-third pair is different; there are two different types of chromosome in this pair, a larger X chromosome, and a smaller Y chromosome. The X chromosome carries many genes, but the Y chromosome carries few genes including the genes with the instructions for being male. Thus, a person with two X chromosomes is a female, while a person with one X and one Y chromosome is a male. For this reason, these are called the sex chromosomes.
Genes that are carried on the X chromosome are called X-linked genes. In a male, since the Y chromosome does not carry the same genes as the X, the genes on the X will not be paired; they will be present in only one copy. In this way, a gene on the X chromosome in a male will be expressed whether or not it is recessive or dominant; there is no other gene to counteract it. Genes in a female with two X chromosomes will be paired, however, and follow the usual rules of dominance and recessive. A recessive gene on one X chromosome will not show up in a female who also carries the dominant gene on the other X chromosome. If she transmits the X with the recessive gene to a son, it will show up in him. There would be a 50% chance that the recessive gene would be passed on. This gives a characteristic pattern of inheritance where only males express the recessive condition, which they inherit through their unaffected mothers. There would be no cases in which a male would pass the condition on to one of his sons (since his sons, in order to be male, would have had to inherit his Y chromosome instead of the X). None of his daughters would show the recessive condition, but they all would carry it on one X and would have a 50-50 chance of passing it on to their children, but only males inheriting the recessive gene would have the condition.
In contrast, an X-linked dominant gene will have a slightly different pattern of inheritance in families. Since the gene is dominant, it will show up no matter what the other gene on the X codes for, so it will show up in a female. If a female has an X-linked dominant condition, both her sons and daughters have a 50-50 chance of inheriting it from her, the same as if it was an autosomal dominant gene.
It is only in the children of males with the condition that we can see that the gene is on the X chromosome. Recall that if a male passes his X chromosome on to a child, the child is female; if he passes the Y on, the child is male. Thus, if the male has the dominant gene on his X chromosome, all of his daughters will have inherited that gene and have the condition it codes for; but his sons will not have the condition and cannot pass it on, since they inherited the Y chromosome.
In real life, the distinction between dominance and recessiveness is usually not black-and-white; or, to continue with the example above, not black and blond. One allele often does not completely dominate the other, and the presence of a different gene may affect the final expression. Thus, a person with one gene for black hair and one for blond may actually have somewhat lighter hair than someone who has both genes coding for black hair. In the same way, for many X-linked dominant medical conditions, females may not be as severely affected as males, since females have the other allele on their other X chromosome to counteract the effects.
An example of a condition that can be inherited in an X-linked dominant fashion is Alport syndrome, which involves progressive hearing loss and progressive kidney problems. If a woman has Alport syndrome, she may only have mild hearing and kidney problems. Each of her children has a 50-50 chance of inheriting the condition, but it will be more severe in her sons. If a man has the X-linked form of Alport syndrome, all of his daughters will also have it, but none of his sons will have it.
Date Originally Created: Fall of 1989
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
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