Genetic Deafness What are genes?

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Genetic Deafness
Genetic deafness is caused by one or more changed genes. Hearing is a delicate and
complex system. Nearly 100 genes are needed to make a normal ear and to hear properly.
Changes or mutations in one of these genes may cause deafness.
What are genes?
Our bodies are made up of millions of cells. Each cell contains a complete set of genes.
We have thousands of genes. We each inherit two copies of most genes, one copy from
our mother and one copy from our father. Genes act like a set of instructions, controlling
our growth and how our bodies work. Any alteration in these instructions is called a
mutation (or change). Mutations (or changes) can stop a gene from working properly. A
mutation (change) in a gene can cause a genetic disorder. Genes are responsible for
many of our characteristics, such as our eye colour, blood type or height.
Genes are carried on thread-like structures called chromosomes. Each of us has 46
chromosomes in every cell. We inherit our chromosomes from our parents, one set of 23
chromosomes from our mother and one set of 23 chromosomes from our father. So we
have two sets of 23 chromosomes, or 23 pairs. One pair of chromosomes are called the
sex chromosomes. The other 22 pairs are called autosomes. Changes (mutations) in
autosomal genes can cause autosomal genetic disorders.
How is Genetic Deafness inherited?
Genetic deafness is usually inherited in one of two ways. The two ways are called
autosomal dominant and autosomal recessive inheritance, and they have different
consequences to the family and the children.
What is Autosomal recessive inheritance?
Recessive genetic deafness is caused by changes (mutations) in a pair of genes. Both
genes must have a change (mutation) for the person to be affected. An affected person
inherits a copy of the gene with a change (mutation) from each parent. In most cases, the
parents of the affected individual are healthy carriers of a single changed copy of the gene.
Carriers can hear because they have one copy of the gene without the change (mutation):
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this copy is working properly. However, a person who has a change (mutation) in both
copies of a recessive gene cannot hear.
What does it mean to be a carrier?
A carrier of an autosomal recessive deafness gene is a person who has one normal copy
of the gene and who has a change (mutation) in the other copy of that gene. The presence
of the normal gene usually ensures that the person will not be affected.
Generally, the chance that someone who carries a gene with a change (mutation) will
choose a partner who happens to carry the same changed (mutated) gene is small
(providing that the partner is not a close relative and does not have a family history of
genetic deafness). We are all carriers of 3 or 4 recessive gene changes without knowing it.
If both partners in a couple are carriers of the same changed gene, they are at risk of
having a child with genetic deafness. The chance that such a couple will have an affected
child is one in four (1 in 4, or 25%) for each pregnancy.
Parents can sometimes misunderstand the 1 in 4 risk. Parents who have one child
affected by recessive deafness may think that a 1 in 4 risk means that the next 3 children
cannot be affected. This is not true. The risk (25% or 1 in 4) is the same for every
pregnancy.
There is also a 2 in 4 chance that a child will inherit a single copy of the gene with the
change (mutation). Children who inherit just one copy of the gene with the change
(mutation) will be healthy carriers like their parents.
This information is summarised in Picture 1.
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Picture 1. How recessive genes are passed on by carriers
These shapes represent a pair of genes
This shape represents a normal copy of the gene
This shape represents a copy of the gene with a mutation (change)
Both parents are carriers; they both have a change (mutation) in one copy of the gene.
They both have one normal copy of the gene. There are four possible ways for the parents
to pass on their genes
Father
unaffected child
neither gene has the
change (mutation)
unaffected child
Mother
unaffected child
These two children are carriers, like their parents
affected child
both genes
have the
change
(mutation)
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What is Autosomal Dominant Inheritance?
Some genetic deafness is caused by a change (mutation) in one copy of an autosomal
gene; the other, normal gene cannot compensate for the effects of the change. This is
called dominant inheritance, as the changed gene dominates the normal gene. Some
people with a dominant change (mutation) in a gene for hearing are profoundly deaf and
others are partially hearing. We do not know why this is.
How are dominant genes inherited?
When affected people have children, they may pass on either the normal gene or the gene
with the mutation (change) to each child. This occurs randomly, so there is a 50%, or 1 in
2, chance that a child will receive a changed copy of the gene and may develop the
condition. This chance remains the same in every pregnancy and is the same for boys or
girls.
There is also a 50%, or 1 in 2, chance that a child will receive the normal copy of the gene.
In this case, the child will not be affected by the disorder and cannot pass it on to any of
his or her children .
This information is summarised in picture 2
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Picture 2. How dominant genes are passed on from parent to child
These shapes represent a pair of genes
This shape represents a normal copy of the gene
This shape represents a copy of the gene with a mutation (change)
There are four possible ways for the parents to pass on their genes
Unaffected parent
unaffected
child
unaffected
child
50% chance that child
will not inherit the changed gene
Affected parent
affected
child
affected
child
50% chance that child
will inherit the changed gene
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What if a child is the first person in a family to have the condition?
Sometimes a child born with dominant genetic deafness can be the first person to be
affected in a family. This can happen when the change (mutation) in the gene arises only
in the egg or sperm of a parent, but is not present in any other type of cell. When this
happens, the parent is not affected, but can pass on the changed gene to children. In this
case, the parents have a very low chance of having another child with the condition.
However the affected child, who has inherited the changed gene, can pass it on to the next
generation.
About 10% of genetic deafness is inherited in this way (Dominant Deafness).
Gene tests and Hearing
Can a test show whether my deafness or my child’s deafness is
genetic?
All the genes for hearing are not yet known. Genetic tests are possible, but there are not
tests for all of the gene changes (mutations) that can cause deafness. There is a test for
one gene called Connexin 26 (or GJB2). This gene is a common cause of recessive
deafness all over the world. So if mutations are found in this gene, it means the person
has genetic deafness (even if no-one else in the family is deaf).
If a person’s Connexin 26 gene test is normal does it mean their
deafness is not genetic?
No.
If the gene test is normal,
either the person’s deafness is caused by a gene change for which there is no test yet. ,
or the person’s deafness is not genetic at all.
So if genetic tests are normal, the chance of a particular couple having deaf children must
be estimated, based on the family medical history of both partners, and on the specialists’
experience of what has happened to other couples in the same situation.
Some people are disappointed if the gene test result is not definite. The amount and
accuracy of the information we can give to families is improving all the time.
Scope of this information
Deafness can be inherited in ways apart from dominant and recessive inheritance, but this
is very rare. If this is thought to be the case in your family, you should discuss it in the
genetic clinic.
This information does not cover the situation where both members of a couple are deaf.
This needs to be discussed individually in the genetic clinic. In some families with many
deaf people in different generations, there may be several deafness genes in the family.
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Where can I find out more?
More information can be obtained from your local regional genetics centre or from these
addresses:-
RNID, Royal National Institute for Deaf and hard of hearing people
19-23 Featherstone Street,
London
EC1Y 8SL
Telephone: 020 7296 8000
Textphone: 020 7296 8001
RNID information lines (Freephone)
Telephone: 0808 808 0123
Textphone: 0808 808 9000
Fax: 020 7296 8199
Email: [email protected]
Web www.rnid.org.uk
The Genetic Interest Group
Unit 4D,
Leroy House,
436 Essex Rd.,
London
N1 3QP
Telephone: 020 7704 3141
Email: [email protected]
Web: www.gig.org.uk
Contact a Family
209-211 City Rd.,
London
EC1V 1JN
Telephone: 020 7608 8700
Fax: 020 7608 8701
Helpline 0808 808 3555 or Textphone 0808 808 3556
(Freephone for parents and families, 10am-4pm, Mon-Fri)
Email: [email protected]
Web: www.cafamily.org.uk
This edition prepared in July 2005
Ref 35
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Glossary (difficult words and their meanings): Genetic Deafness
This glossary is intended only to explain terms used in the information: Genetic Deafness.
Words shown in bold are defined elsewhere in the glossary.
autosomal. Involving the autosomes.
autosomal dominant genetic disorders. Inherited disorders caused by a change
(mutation) in one copy of an autosomal gene.
autosomal recessive genetic disorders. Disorders caused when a person inherits
two copies of an autosomal gene with a change (mutation) in both copies. A person who
has only one copy of that autosomal recessive gene with the change (mutation) will be an
unaffected carrier. A person who is affected with an autosomal recessive disorder inherits
one copy of the disease gene with the change (mutation) from each parent.
autosomes. The 44 chromosomes (22 pairs) which are not sex chromosomes.
carrier. Someone who has one normal copy of an autosomal recessive gene and who
has a change (mutation) in the other copy of that gene. The presence of the normal gene
usually ensures that a carrier is not affected by the corresponding autosomal recessive
genetic disorder.
cell . The human body is made up of millions of cells, which are like building blocks. There
are many specialised types of cells. These include skin cells, brain cells, and blood cells.
Cells in different parts of the body look different and do different things. Every cell (except
for eggs in women and sperm in men) contains all the body’s genes.
chromosomes. Thread-like structures which can be seen under the microscope and
contain the genes. Usually people have 46 chromosomes in every cell. There are two sex
chromosomes. The other 22 pairs of chromosomes (numbered 1 to 22) are called
autosomes. Twenty-three chromosomes come from the mother, and twenty-three come
from the father. One chromosome of each pair comes from each parent. (As an analogy: a
chromosome is like a book; a gene is like a story in the book).
egg. The mother’s contribution to the cell which will grow to make a new baby. The egg
contains 23 chromosomes; one from each pair in the mother. A sperm also contains 23
chromosomes one from each of the father’s pairs. The egg joins with a sperm to make a
complete cell. A baby develops from this first cell.
gene. Information needed for the body to work, stored in a chemical form on
chromosomes. Changes or mutations in genes alter the information and this can
change how the body works. Autosomal genes are in pairs: one from the mother, one
from the father. The two genes of a pair are at matching places on a pair of
chromosomes. (As an analogy: a chromosome is like a book, a gene is like a story in
the book, a change or mutation in a gene is like a missing or extra letter in a word in the
story).
genetic. Caused by genes, concerning genes.
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mutation. A change in a gene. Some mutations are not harmful. Sometimes when a
gene is changed, its information is altered so it does not work properly. (As an analogy: a
change or mutation in a gene is like a missing or extra letter in a word in a story).
sex chromosomes. The X chromosome and the Y chromosome. The sex
chromosomes control whether a person is male or female. Females have two X
chromosomes. Males have one X and one Y chromosome.
sperm. The father’s contribution to the cell which will grow to form a new baby. Each
sperm contains 23 chromosomes; one from each pair in the father. An egg contains one
chromosome from each of the mother’s 23 pairs. The sperm joins with an egg to make a
complete cell. A baby develops from this first cell.
This glossary is intended only for use by patients and families, with the genetic information
to which it refers.
This edition prepared in July 2005
Ref Glossary 35
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