About 1 in 500 infants is born with or develops hearing loss 

during early childhood. Hearing loss has many causes: some are 

environmental (such as certain infections in the mother during 

pregnancy, or infections in the newborn baby), and some are 

genetic. A combination of environmental factors and genes also 

could cause hearing loss. For many babies, the cause of hearing 

loss is unknown.

This booklet answers many of the questions that families have about 

the causes of hearing loss, including genetic causes. For some of 

the topics, more extensive details are given in the shaded boxes.


About The Causes of Hearing Loss

What Are The Types of Hearing Loss?

► One way is based on whether or not the baby was born 

with hearing loss. (congenital vs. acquired)

► Another way depends on whether or not the hearing loss 

gets worse over time. (progressive vs. nonprogressive) 

► A third way depends on whether or not other symptoms are 

present. (syndromic vs. nonsyndromic)

► A fourth way depends on whether or not hearing loss runs 

in the family. (familial vs. sporadic)

► A fifth way is based on where in the ear the hearing loss 

occurs. (conductive, sensorineural or mixed)

Drawing by: S. Blatrix

© Centre Régional d’Imagerie Cellulaire. Used with persmission.

www.iurc.montp.inserm.fr/cric/audition/english/ear/fear.htm 

A Diagram of the External (E), 

Middle (M), and Inner (I) Ear


One way to describe hearing loss is as “congenital” or “acquired”. 

Congenital means that a person was born with the hearing loss. Babies 

born with hearing loss can be identified through a newborn hearing 

screening test. The test is often done before a baby leaves the hospital. 

The screening test does not tell you the cause of the baby’s hearing loss. 

It can only tell you whether the baby might have a hearing loss or does 

not have a hearing loss at birth. Acquired hearing loss means that a 

person could hear when he or she was born, but developed hearing loss 

later in life. Acquired hearing loss can also be described by the age at 

which it starts. If hearing loss starts before the age when children usually 

begin talking, it is called “pre-lingual”, which means “before speaking”. If 

hearing loss starts after the age when children begin talking, it is called 

“post-lingual”, which means “after speaking”. 

If the hearing loss gets worse over time, it is called “progressive”. If the 

hearing loss does not change over time, it is “nonprogressive” or stable.

“Syndromic” means that a person has other symptoms besides hearing 

loss. For example, some people with hearing loss are also blind. There 

are many different syndromes that have hearing loss as one of the 

symptoms. “Nonsyndromic” means that the person does not have any 

other symptoms. 

If more than one person in a family has hearing loss, it is said to be 

“familial”. That is, it runs in the family. If only one person in the family 

has hearing loss, it is called “sporadic”. That is, it does not run in the 

family. 

The middle ear contains small bones that help send sound from the air 

to the inner ear. The inner ear changes these sounds into nerve signals 

that go to the brain. “Conductive” hearing loss is caused by problems 

in the external or middle ear. “Sensorineural” hearing loss is caused by 

problems in the inner ear or in the hearing nerve. “Mixed” hearing loss is 

due to both kinds of problems.


What Causes Hearing Loss?

About 1 in 500 infants is born with hearing loss or develops hearing 

loss in early childhood. Hearing loss can be caused by changes in 

genes or the environment, or both. For many children with hearing 

loss, the cause is unknown.

Genes are passed from parents to children and cause things to run in 

families. Genes are described in more detail in the next section. There 

are many genes that are involved in hearing. Sometimes, a gene does 

not form in the expected manner. This is called a mutation. Some 

mutations cause syndromic hearing loss and others cause nonsyndromic 

hearing loss. In some families with genetic hearing loss, the loss is not 

due to mutations in any of the known genes. This means that scientists 

are working to find all of the genes involved in hearing loss.

There are also a number of things in the environment that can cause 

hearing loss. For example, babies who are born too early and babies 

who need help breathing (for instance, using a ventilator) are more likely 

to develop hearing loss than are other babies. Some infections (such as 

cytomegalovirus) in the mother during pregnancy can cause the baby to 

have hearing loss. Also, some infections (such as meningitis) in young 

babies can cause hearing loss.

Sometimes, both genes and environment work together to cause hearing 

loss. For example, there are some medicines that can cause hearing 

loss, but only in people who have certain mutations in their genes.


How Do Doctors Figure Out What 

Caused a Person’s Hearing Loss?

Doctors begin by looking at the person’s physical features, 

medical history, and family history. Based on this, they 

classify the hearing loss in the ways described earlier 

(congenital or acquired, prelingual or postlingual, progressive 

or nonprogressive, conductive or sensorineural, syndromic or 

nonsyndromic, and familial or sporadic). The classifications 

often point to certain causes. The doctors might ask for more 

medical tests to look for symptoms of syndromic hearing loss, 

and they might ask for genetic tests.


About Genes & Mutations

What Are Genes?

Genes contain the instructions that tell the cells of people’s bodies 

how to grow and work. For example, the instructions in genes 

control what color a person’s eyes will be. Each person gets half of 

his or her genes from each parent, and that is why a person tends to 

look like his or her parent.

Genes are stored in a chemical called deoxyribonucleic acid (DNA). DNA

is a chemical chain made up of “bases”. There are four bases: A, C, T 

and G. The specific order, or sequence, of these bases determines the 

exact information carried in each gene, like the way that a specific pattern 

of letters makes up the words in a sentence.

The DNA is packaged into small units called chromosomes. Each cell 

in a person’s body contains a set of chromosomes, and thus a set of 

genetic instructions. Every person should have 23 pairs of chromosomes. 

One chromosome from each pair is from the person’s mother and one 

chromosome of each pair is from the father. 

The first 22 pairs of chromosomes are the same in everybody. The 23rd 

pair is called the sex chromosomes, and they help determine if a person 

is male or female. A female has two X chromosomes, and a male has 

one X and one Y chromosome. A mother will give one of her two X 

chromosomes to each of her children. A father will give either his X or his 

Y chromosome. If the child gets his father’s Y chromosome, he will be a 

boy. If the child gets her father’s X chromosome, she will be a girl.


The Relationship Between 

Cells, Chromosomes, DNA, 

Bases, and Genes


The figure above shows how children get their chromosomes, and 

therefore their genes, from their parents. In this figure, three pairs of 

chromosomes are shown: pair #1 (green), pair #2 (yellow), and the sex 

chromosomes (pink and blue). The father’s chromosomes are shown in 

solid color, and the mother’s are striped. Children randomly get one of 

each pair of chromosomes from their mother (striped) and one of each 

pair from their father (solid). Each daughter gets an X from her mother 

(striped) and an X from her father (solid). Each son gets an X from his 

mother (striped) and a Y from his father (solid).

An Example of How Chromosomes Are 

Passed From Parents To Children


What are Mutations?

A mutation is any change in a gene that makes it different from the 

usual copy (the copy that most people have). If the change causes 

a difference in the way that the gene works, the person with the 

mutation may have a particular condition (such as hearing loss) that 

runs in the family. Different changes run in families in different ways, 

as described on the next few pages. 

A mutation is any change in the usual DNA sequence. For example, 

suppose part of a gene usually has the sequence GTAC. A mutation 

can change the sequence to GTTC in some people. This change in 

sequence can change the way that the gene works so that people with 

this mutation can have a particular condition. Some mutations do not 

seem to affect the person who has them.

Genetic conditions are often described in terms of the chromosome 

that contains the gene. If the gene is on one of the first 22 pairs of 

chromosomes, called the autosomes, the genetic condition is called an 

“autosomal” condition. If the gene is on the X chromosome, the condition 

is called “X-linked” or “sex-linked”.

Genetic conditions are also grouped by how they run in families. 

Different mutations cause conditions to run in families with different 

patterns. Mutations are called “dominant” or “recessive” depending on 

how they cause conditions and how they run in families. 
10

Autosomal Dominant Conditions

“Autosomal” conditions affect both males and females equally. In 

“dominant” conditions, the condition is passed from parent to child. 

If one parent has the condition, each child has a 50% (1 in 2) chance 

of having it as well.

“Autosomal” genes are genes that are found on one of the first 22 pairs of

chromosomes, the pairs that are the same for both males and females. In 

the case of “dominant” conditions, a child will have the condition if he or 

she has one copy of the gene with the dominant mutation. It takes only 

one copy of a dominant mutation to cause the condition.

The figure on the next page shows how children get dominant conditions 

from their parents. In this example, the usual copy is symbolized by 

and the copy with the dominant mutation is symbolized by . When one 

parent has the dominant condition, he or she has a usual copy ( ) and 

a copy with the dominant mutation ( ). He or she will give each child 

either the usual copy ( ) or the copy with the dominant mutation ( ). 

Therefore each child has a 50% (1 in 2) chance of getting the gene with 

the dominant mutation and therefore having the condition. If the other 

parent has two usual copies, and therefore does not have the condition, 

he or she will give each child one of the two usual copies ( ). Even 

though a child gets one of the usual copies from the parent who does not 

have the condition, if he or she gets a dominant copy from the parent with 

the condition, the child will also have the condition.
11

An Example of Autosomal 

Dominant Inheritance

Deaf

Father

Deaf

Daughter

Hearing

Son

Deaf

Son

Hearing

Daughter

Hearing

Mother

copy with a dominant mutation.
An Example of Autosomal 

Recessive Inheritance

12

Hearing

FatherCarrier

Hearing

ChildNoncarrier

Hearing

ChildCarrier

Hearing

ChildCarrier

Deaf

ChildCarrier

Hearing

MotherCarrier

copy with a recessive mutation
13

Autosomal Recessive Conditions

“Autosomal” conditions affect both males and females equally. 

“Recessive” conditions are also due to changes in genes, but they 

run in families in a different way than dominant conditions. Usually, 

one or more children with the condition will be born to parents who 

do not have the condition. This is because people who have one 

copy of a recessive mutation do not have the condition. They are 

called “carriers”. If two carriers have a child together, there is a 25% 

(1 in 4) chance that the child will get two copies of the mutation and 

therefore have the condition.

“Autosomal” genes are genes that are found on one of the first 22 pairs 

of chromosomes, the pairs that are the same for both males and females. 

In the case of “recessive” conditions, a child will have the condition only if 

he or she does not have a usual copy of the gene.

The figure on the left shows how recessive conditions run in families. 

In this example, each parent is a carrier and has one usual copy 

(symbolized by ) and one copy with a recessive mutation (symbolized 

by ). Each parent will give the child either the usual copy ( ) or the 

copy with the recessive mutation ( ). Each has a 50% (1 in 2) chance of 

happening. If the child gets one usual copy ( ) from one parent and one 

copy with the recessive mutation ( ) from the other parent, the child will 

be a carrier like both parents. If the child gets the copy with the recessive 

mutation ( ) from both parents, and therefore doesn’t have a usual copy, 

the child will have the condition. When both parents are carriers, there 

is a 25% (1 in 4) chance that each child will get a copy with a recessive 

mutation from both parents and, therefore, have the condition.
14

X-Linked Recessive Conditions

X-linked conditions usually only affect males. The mutation can be 

passed in the family through female carriers who do not have the 

condition. However, each son of a female carrier has a 50% chance 

of inheriting the mutation and therefore having the condition.

“X-linked” genes are genes that are found on the X chromosome. 

“Recessive” means that a person has the condition only if he or she does 

not have at least one usual copy of the gene. 

A female has two X chromosomes and, therefore, two copies of each 

X-linked gene. A woman who has one usual copy and one copy with 

a recessive mutation is called a carrier. A carrier does not have the 

condition, but can pass the copy with the recessive mutation on to her 

children. 

A male has only one copy of the X chromosome, which he has gotten 

from his mother. His other chromosome is a Y chromosome that he 

gets from his father. Therefore, a male has only one copy of each 

X-linked gene. If the male gets his mother’s X chromosome that 

has the recessive mutation, he will have the condition. If he gets his 

mother’s X chromosome that has the usual copy of the gene, he will 

not have the condition. Therefore, a son of a carrier mother has a 

50% chance of having the condition. Since a male has only one X 

chromosome and therefore he must pass that X chromosome to each 

of his daughters, each daughter will be a carrier. Since a male has 

only one Y chromosome, he will pass that Y chromosome (and not the 

X chromosome) to each of his sons, so none of his sons will have the 

condition or be carriers.
15

An Example of X-linked 

Recessive Inheritance

Hearing

Father

Hearing

DaughterNoncarrier

Hearing

Son

Hearing

DaughterCarrier

Deaf

Son

Hearing

MotherCarrier

usual copy of the gene, and is an X 

chromosome with a copy with a recessive

mutation. is a Y chromosome.
16

What are GJB2 and Connexin 26?

A recent and exciting discovery is the GJB2 gene. The GJB2 gene 

contains the instructions for a protein called connexin 26; this protein 

plays an important role in the functioning of a part of the ear called the 

cochlea. The cochlea is a very complex and specialized part of the body 

that needs many instructions to guide its development and function. 

These instructions come from genes. Changes in any one of these genes 

can result in hearing loss. 

In some populations about 40% of newborns with a genetic hearing loss 

who do not have a syndrome, have a mutation in the GJB2 gene. There 

are many different mutations in this gene that cause hearing loss. Most 

of these mutations are called recessive, meaning that a person can have 

one usual copy of the gene and one copy of the mutation and will have 

normal hearing function. (Every one has two copies of the GJB2 gene, 

one from each of their parents). However, a person who has two copies 

of a gene with a mutation, one mutation inherited from each parent, 

will have hearing loss. This means that if both parents have a copy of 

the gene with a mutation, they can have a child with hearing loss, even 

though both parents can hear. In fact, 90% of babies with hearing loss 

are born to parents who can hear.
17

Multifactorial Conditions

Sometimes conditions are caused by a combination of genes and 

other factors. These conditions are said to be “multifactorial”. If their 

first child has a multifactorial condition, the parents’ chance of having 

a second child with the condition depends on the specific condition, 

and is usually about 3% to 5%. 

People who have multifactorial conditions are often born into 

families with no other affected people. Parents of a child with any 

such condition have a greater chance of having another child with 

the same condition than parents who do not have a child with the 

condition. 

The Relationship Between Chromosomes, 

Mitochondria, and Mitochondrial DNA
18

Mitochondrial Conditions

“Mitochondrial” conditions are different from most other genetic 

conditions because in this case, only the mother can pass it on. If a 

woman has a mitochondrial condition, the chance that she will pass 

it on to her children depends on the particular condition and on how 

severely she is affected. Fathers with a mitocondrial condition do 

not pass it on to their children.

Not all of a person’s genes can be found on their chromosomes. A few 

genes are found on small, circular pieces of DNA in the mitochondria, tiny 

parts of cells that make energy. These genes are called “mitochondrial” 

genes. Each cell has many mitochondria and, therefore, many copies 

of each mitochondrial gene. The chance that a person will have a 

mitochodrial condition depends on the number of his or her mitochondria 

that carry a mutation. Mitochondrial genes are passed on to children 

differently from the way that genes on chromosomes are passed on 

because each person gets his or her mitochondrial genes only from his 

or her mother. Therefore, if a woman carries a variation in one of her 

mitochondrial genes, each child has a chance of having the condition 

or of being an unaffected carrier. The exact chance depends on the 

number of copies with mutations that the mother is carrying. A male 

with a mitochondrial condition will not pass the condition to his children, 

because males do not pass their mitochondrial genes to their children.

The figure on the previous page shows how a fertilized egg (which 

will grow into a baby) gets chromosomes from the mother’s egg and 

the father’s sperm (shown as red and blue chromosomes), but gets 

mitochondrial DNA only from the mother’s egg.
19

About Genetic Testing

What Is Genetic Testing?

A genetic test involves looking at a person’s 

DNA to see if certain mutations are present. A 

person’s DNA sample is usually gotten from 

one of two different sources: (1) a small sample 

of a person’s blood, or (2) cheek cells from a 

person’s mouth. To get the cheek cells, a small, 

toothbrush-like swab is rubbed inside a person’s 

mouth. The cheek swab is easy and painless, 

but the DNA obtained from this method is 

sometimes unstable and might not be usable. 

Once a person’s DNA sample is obtained, there are 

two different ways to look for mutations. The first 

way looks only for certain mutations. This “mutationspecific testing” will detect that one type of mutation 

if it is present, but it will not detect other mutations that might also be 

present. This type of testing is used when most of the people who have a 

genetic condition have the same mutation.

The other type of genetic testing is called “sequencing”. In this method, 

the DNA sequence is determined for the entire gene or for a certain part 

of it. This method will detect any mutation that is present in the part of 

the gene looked at. This type of testing is harder and costs more. It is 

usually used when any one of several mutations in a gene could cause a 

condition.
20

What Are the Benefits of Genetic Testing?

If a mutation is found, it may explain why the person has a condition 

such as hearing loss. In some cases, knowing what mutation a 

person has will allow doctors to predict how severe the condition 

might become and what other symptoms can be expected. Then, 

the person may get any other medical care that might be needed. 

Also, knowing the cause of a person’s condition will let them know 

what the chances are of passing the condition on to their children. 

It also lets other family members know the chances that they might 

have a child with the same condition. For example, knowing the 

cause of a child’s hearing loss will let the parents know the chances 

of having another child with hearing loss if they have another child 

together in the future. Prenatal testing (testing a baby before it is 

born to see if it has the mutation) might also be possible.

What Are the Limits of Genetic Testing?

► Not all of the genes that cause conditions are known. 

So even if a condition runs in the family, it might not be 

possible to find the mutation that caused it. 

► Some tests are hard to do. For example, the bigger a 

gene is, the harder it is to study the whole gene.

► Sometimes it is not possible to tell if a mutation is the 

cause of the condition, or just a coincidence.

Some mutations cause conditions in most people, but not in all people 

who carry the mutation. For these mutations, a positive result (that is, 

results showing that a person has one or more copies of the mutation) 

might not mean that the person will get the condition. Likewise, a 

negative result (that is, results showing that a person does not carry a 

copy with that mutation) does not guarantee that the person will not get 

the condition. This is because the person might have a different mutation 

that was not detectable by the test used, or the person might have a 

mutation in a different gene that also causes the same condition.
21

What Are the Risks of Genetic Testing?

Some people have strong feelings when they get the results of their 

genetic test. Some people feel angry, sad, or guilty if they find out 

that they or their child have a mutation. It is important to remember 

that everyone carries mutations of some kind, and that a person’s 

genes are no one’s “fault”.

Genetic tests are different from other medical tests in that the results 

provide information about other members of the family, and not just 

the person being tested. Some family members do not want to know 

that a mutation runs in their family. Also, because children get their 

genes from their parents, genetic tests that involve several family 

members can reveal personal information, such as a child having 

been adopted or having a different biological father.

Sometimes, people are concerned about keeping the results of their 

genetic test private. For example, they do not want their friends, 

relatives, or coworkers to find out. Companies that offer genetic 

testing are very careful to make sure that test results are kept 

private. Test results cannot be seen by anyone who is not involved 

in the testing, unless the person tested or his or her parents or 

guardians give permission. 
22

Where Can I Find More Information?

For more information about genes related to hearing loss and about 

genetic testing, contact a genetic counselor in your area.

For more information about hearing and hearing loss, see the 

National Institute on Deafness and Other Communication Disorders 

at http://www.nidcd.nih.gov/health/hearing/

This guide from the U.S. Department of Health and Human Services 

explains about genes and genetic testing.

http://www.accessexcellence.org/AE/AEPC/NIH/index.html

The Genetic Science Learning Center has some information about 

basic genetics, genetic conditions and genetic counseling. 

http://gslc.genetics.utah.edu/

This source from Cold Spring Harbor Labs is a tutorial about 

genetics. It is a little more in-depth than some of the other sites. 

http://www.dnaftb.org/dnaftb/

For more information on Early Hearing Detection and Intervention 

see http://www.cdc.gov/ncbddd/ehdi/default.htm



DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Center on Birth Defects and Developmental Disabilities

Division of Human Development and Disability

Early Hearing Detection and Intervention

1600 Clifton Road

Atlanta, GA 30333

CS112691