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The Biology of Deafness

Employment Matters column, i711.com


According to the biology departments at Gallaudet University and the Medical College of Virgina, at least 85% of deaf people will marry another deaf person.

Many of those marriages will eventually include children, and if their parents became deaf by genetic causes, there’s certainly a chance of the children being deaf too. Among the general population, about 1 in 1,000 children in the US is born profoundly deaf.

About half of all deaf people are genetically deaf, and the other half became deaf through environmental causes. Environmental causes include premature birth, viruses that reach the fetus, and certain drugs that can affect the fetus.

Genes come with on/off switches, so not all genes are working all the time. Some genes are only read during fetal development, others during puberty, and others can be on or off depending on the activity of other genes or the environment.

The genes that cause deafness can be either dominant or recessive. If a gene is dominant, it will be active when only one copy of the gene is present. Recessive genes are only active when two copies are present.

Everyone gets genes from both parents. Dominant genes for deafness can come from either parent, but a recessive gene for deafness means both parents must pass it on for that gene to be active.

Most genetically deaf people inherit recessive genes, which is why deafness can sometimes skip generations – childen may get only one recessive gene from their parents, and so become carriers of the deafness gene, but they won’t be deaf themselves.

Working out how genes interact with each other and the environment is an extremely complex subject, and there are at least several hundred genetic changes that can lead to deafness. But just a few common causes dominate the list. You probably know of some of them, either from personal experience or through people you know.

Genetic causes for deafness are grouped in two ways. One group is linked with other traits, or syndromes. In the second group, deafness appears unrelated to anything else.

The first, syndromic group, include Usher’s, Waardenburg and Pendred syndrome. One feature of Waardenburg is a forelock of white hair, so if you’ve seen that, you’ve seen an example of syndromic deafness. These forms of deafness arise when mutations in a gene or a group of genes affect more than just hearing.

Genes provide instructions – you can think of it as a recipe – for making proteins. Our bodies use some proteins for more than one purpose. If a mutation affects one of these multi-purpose proteins, the effect shows up as a syndrome. Different areas of the body feel the impact of the missing or incomplete protein.

There are also at least 30 genetic causes of deafness that are not apparent through appearance or other issues. Most deaf people – 70 to 80% – are in this group. Of the major causes in this group is a mutation of the GJB2 gene.

This gene makes a protein called connexin 26, which makes a critical part need by cells to communicate with neighboring cells. This communication trouble at the cellular level scales all the way up to the whole individual!

Many genes in both groups have been mapped – their locations are precisely known, and tests for some are available. Some genes are so large that testing them for deafness-related mutations is too expensive, but among the smaller and well-known deafness genes, it’s possible to arrange tests. There are still some genetic causes of deafness where the location of the gene remains a mystery.

Both genes and our environment have a powerful influence on the people we become. But in the end, how we play the environmental and genetic cards we’re dealt has the greatest impact on our lives. The actions we take and the choices we make define us more than anything else.

Related Links:

Hereditary Hearing Loss Homepage

Deafness and Hereditary Hearing Loss Overview

Chart of Inheritance Percentages

Basic deaf-related Genetics

Nonsyndromic Deafness

Finding Genes for Non-Syndromic Deafness

Deafness-related Gene Testing

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Written by Influential Prose

September 30, 2009 at 11:54 pm

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