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Deafness
Inheritance patterns and prenatal diagnosis
About 50% of all PCHI is due to a genetic cause. Of this 75-80% show an autosomal recessive (AR) inheritance. In this form of inheritance each parent passes on one copy of the gene responsible for deafness and therefore the baby has two copies of the same gene thus manifesting the condition while the parents are asymptomatic as they also have a copy of the normal gene. AR hearing loss can be part of a syndrome such as Usher syndrome and Pendred syndrome, or non-syndromic (NS). The commonest AR gene responsible for non syndromic deafness (NS) is Connexin 26. This accounts for 40-50% of all AR NS deafness and can be tested for.
About 20% of PCHI is due to an autosomal dominant (AD) inheritance, where the child needs only one copy of the dominant gene to be affected. Often there is deafness on one side of the family with each baby carrying approximately 50% chance of having the dominant gene responsible for deafness. Examples of dominant syndromic deafness include Treacher Collins syndrome, Branchio Otorenal (BOR) syndrome and Waardenburg syndrome. Often in AD syndromic hearing loss the degree of symptoms and signs may vary from member to member because of variability of penetrance and expressivity of the gene responsible.
Other forms of inheritance include X-linked and mitochondrial. Inheritance of X-linked deafness e.g. Alport syndrome (hearing loss and kidney involvement) is from affected father to daughters (because the father passes on the Y chromosome to his sons) and from affected mother to approximately 50% of all children irrespective of the sex. As mitochondria are in the cytoplasm, and it is only the ovum that contains cytoplasm (sperm has only the nucleus and no cytoplasm) the inheritance of mitochondrial deafness is from mother to children. Often in mitochondrial deafness there is more than one system/ organ involved. A1555G is one such mitochondrial mutation that makes individuals more susceptible to ototoxicity from aminoglycoside drugs such as gentamicin.
It must be remembered that more than one gene could be responsible for a given syndrome and some syndromes may show more than one pattern of inheritance.
Your geneticist should be able to tell you more about the small number of genes that can be tested for, but as new mutations are identified it is expected that this number will increase.
Prenatal diagnosis
Where deafness is part of a specific syndrome or condition with major abnormalities prenatal diagnosis may be available.
How is it treated
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