How genes can cause deafness

The most common way deafness is passed down in families is when two parents each pass on a recessive gene. This is known as autosomal recessive inheritance (‘autosomal’ means that the gene is located on a pair of the non-sex chromosomes).

Many genes work in a recessive way. This means that, as long as someone has at least one working copy of that specific gene pair, the cell produces enough working protein so the variant (‘non-working’) gene is effectively ‘recessive’ or ‘hidden’. It’s estimated that we all carry some non-working recessive genes, but because we have a second working copy of the gene, we don’t notice any effects.

To give you a visual picture, imagine you're making a chocolate cake. You have two copies of the recipe: one copy has cocoa in it, but the other copy has the cocoa ingredient missing. If the two mixtures are put together in the same bowl, there will still be enough cocoa to give a chocolate flavour. However, if both copies are missing the cocoa, the cake will be different.

If both parents are carriers of one recessive gene variant that causes deafness, but each have a second working copy, they will not have deafness due to this gene. Each time they have a child, each of them could pass on either the working gene copy or the non-working copy.

• There’s a 1 in 4 (25%) chance their child will inherit both non-working genes and have some level of deafness.
• There’s a 1 in 4 (25%) chance their child won’t inherit any copies of the non-working gene, in which case they will not be deaf due to that gene.
• There’s a 1 in 2 (50%) chance their child will inherit one non-working copy and be a carrier like their parents, but not be deaf themselves due to that single altered copy of the gene.

The most common inherited cause of deafness is the Connexin 26 (GJB2) gene, which usually works in a recessive way. Until recently, the only genetic test routinely offered to families with a history of deafness was for Connexin 26.

Read Jessica’s story

Jessica (9) and her parents were given genetic tests after she was identified as being profoundly deaf. The tests revealed her deafness was caused by a mutation of the Connexin 26 gene. Read Jessica's story.

Sometimes, a gene variation can be dominant. This means that one altered copy can cause an effect or condition even if there’s a second working copy of the gene. This is known as autosomal dominant inheritance (‘autosomal’ means that the gene is located on a pair of the non-sex chromosomes).

To give you a visual picture, imagine you’re making potato soup. You have two copies of the recipe, but one has a variation that says tomato instead of potato. Although one of the copies is the original potato recipe, if the two soups are mixed together, the tomato in the altered recipe is enough to give a tomato flavour to the soup.

Dominant conditions can vary in how they affect people. Different people, even within the same family, may have different features of the condition. Some people can carry a dominant gene variation without showing any signs of the condition at all.

If only one parent carries a dominant gene variant that causes deafness, there’s a 1 in 2 (50%) chance that their child will inherit that version of the gene. However, the parent and the child might have different levels of deafness, or one may have no deafness at all.

While most of our genes are carried in the nucleus of the cell, a few are located within the mitochondria. Mitochondria are the powerhouses of the cell: little bodies within the cell that generate the energy that the cells need to function. There are many mitochondria in each cell, each with one copy of their set of genes.

Mitochondria can only be passed on by the mother because the egg is very big compared to the sperm and contains some mitochondria. This means that if a woman carries a gene variant in all or some of her mitochondria, she will pass on this copy to all her children. This is called mitochondrial inheritance. Because the number of mitochondria that inherit the gene variant is variable, the effect of the gene variant will also be variable.

If a dad has a mitochondrial condition, he can’t pass this on to his children.

Conditions with the mitochondrial pattern of inheritance are rare, but certain forms of deafness can be inherited in this way.

Find out more about mitochondrial inheritance from the Genomics Education Programme’s article ‘Meet the mitochondria’.

Genes carried on the X chromosome can affect men and women differently. This is known as X-linked inheritance.

22 of our chromosome pairs are carried by both men and women (our autosomes), but the 23rd pair are our sex chromosomes. Our sex chromosomes determine whether we’re born male or female. Women typically have two long X chromosomes while men typically have one X and one Y chromosome. The Y chromosome is much shorter with very few genes.

Recessive X-linked gene variants

If a woman carries a recessive variant gene on her X chromosomes, her second working gene copy typically compensates mostly or entirely for the non-working copy. This means she will either have no signs of the condition caused by the gene or will have milder signs. However, she is a carrier of that X-linked condition.

If a man has a gene variant on his X chromosome, then he will have the condition as he has only one X chromosome and therefore only one copy of that gene.

If the mother carries a recessive X-linked variant gene

If a woman who’s a carrier of an X-linked condition has a child, the effect of the condition on her child will depend on their sex:

• If she has a boy, there is a 1 in 2 (50%) chance that the boy will inherit her variant copy of the gene and have the condition.
• If she has a girl, there is a 1 in 2 (50%) chance that her daughter will inherit the variant copy and be a carrier like her mother.

Watch this video for an illustration of this situation: X-linked recessive inheritance where the mother is a carrier for a condition.

If the father carries a recessive X-linked variant gene

A father determines the sex of his child because he either passes on an X chromosome (and has a daughter) or a Y chromosome (and has a son), whereas all children inherit an X chromosome from their mother. This means if a man has an X-linked condition and has children, all his daughters will inherit the altered gene copy from him and be carriers. Because a man does not pass on his X chromosome to his sons, none of his sons will have the same X-linked condition.

Watch this video for an illustration of this situation: X-linked recessive inheritance where the father has a condition.

Dominant X-linked variant genes

A few conditions are caused by a dominant gene variant on the X chromosome. In this case, both men and women can show signs of the condition if they inherit a variant copy. For many of these conditions, men are still affected much more than women.

There are both X-linked recessive and dominant genes that can cause deafness.