Aliases: hereditary deafness
While deafness is not a disease (it’s a medical condition), hereditary deafness offers an excellent example of genetic inheritance, making it an ideal topic for this June’s theme, family. There is an obvious connection between an inherited condition and kinship. After all, you share your genes with your relatives. However, there are also more subtle ties between the two. Inheritance can happen in a number of different ways (see the Cause section below), and the pattern of inheritance can have a profound effect on the way a condition is treated by family members and the society at large.
There are many populations that have high levels of hereditary deafness. They usually occur in geographically or culturally isolated communities, where the gene flow (movement of genes) is limited3. The most iconic example may be Martha’s Vineyard. Hereditary deafness reached Martha’s Vineyard in the 17th century during colonization and radiated through the population until 1 out of every 155 (0.7%) islanders were born Deaf, nearly 20 times the US national rate2. This increase reached its peak in the town of Chilmark, where the 1 out of every 4 people were born Deaf1.
A recessive gene was the most likely cause of the hereditary deafness on Martha’s Vineyard. Unlike dominant traits, which only require one copy of the gene, and are passed down directly in families, recessive traits require two copies, one from each parent. The recessive pattern of inheritance is indirect, diffusing the trait across the family tree, making Deaf people less likely to have Deaf immediate family members (although likely to have Deaf people in their extended family). Because nuclear families often included both Deaf and hearing people, Martha’s Vineyard became a bilingual society, with most islanders communicating comfortably in both English and sign language2.
This social integration led to 3 times as many marriages between Deaf and hearing people on Martha’s Vineyard than off the island2. In turn, these families contributed to 250 years of assimilation4, creating a “shared signing community”1 that outlasted hereditary deafness on the island. The high concentration of deafness quickly dissipated in the twentieth century4, when better transportation and communication with the mainland caused a higher rate of immigration to the island, and the last hereditary Deaf person on Martha’s Vineyard died in the 1950s4. But despite the lack of necessity, sign language continued to be used well into the 1980s1. I guess some ties bind.
Cause: Hereditary deafness can be conductive (caused by abnormalities of the external ear or the ossicles of the middle ear), sensorineural (caused by a malfunction of inner ear structure), or a combination. Deafness can be passed down by several types of inheritance: it can be autosomal recessive (requiring the same gene from both mother and father to result in deafness) autosomal dominant (only requiring one copy of the gene, passed directly through generations), X-linked recessive (carried on the X chromosome) or mitochondrial (from a gene in the mitochondria, only passed through the mother). Non-syndromic deafness (has no associated medical disorders) is usually caused by a mutation in one gene and has fairly high heritability (likelihood of being inherited), depending on the type of inheritance (dominant: 75-80%, recessive: 20-25%, X-linked: 1-1.5%)5. By 2001, 11 genes had been identified for autosomal dominant non-syndromic deafness alone6, and more than 400 genetic syndromes are associated with hearing loss5.
Consequence: While hereditary deafness can be syndromic (associated with malformations of the ear or other medical disorders), 80% of genetic deafness is non-syndromic6. Hearing loss may be pre-lingual (before speech develops) or post-lingual5.
Cure: Hereditary deafness is identified with both physical and genetic testing. Because deafness is not considered a disease, a “cure” is not the appropriate way to discuss the treatment of the Deaf. There are many management options for deafness, ranging from hearing aids to cochlear implants5.
1. Kusters, A. Deaf utopias? Reviewing the sociocultural literature on the world’s “Martha’s Vineyard situations”. Journal of Deaf Studies and Deaf Education, 15(1):3-16.
2. Lane, HL, R Pillard, & M French. Origins of the American Deaf-world: assimilating and differentiating societies and their relation to genetic patterning. Sign Language Studies, 1(1):17-44.
3. Marscheck, M & PE Spencer, eds. Oxford Handbook of Deaf Studies, Language, and Education. New York: Oxford University Press, 2003. Print.
4. Novakovic, J. Hereditary deafness on Martha’s Vineyard collection. Martha’s Vineyard Museum. Web. 15 June 2014. http://www.marthasvineyardhistory.org/collections/fa_pdfs/RU%20310–Hereditary%20Deafness.pdf
5. Smith, RJH, AE Shearer, MS Hildebrand, & G Van Camp. Deafness and hereditary hearing loss overview. Gene Reviews. 9 January 2014. Web. 15 June 2014. http://www.ncbi.nlm.nih.gov/books/NBK1434/
6. Zhao, HB. Congenital deafness. American Hearing Research Foundation. March 2007. Web. 4 June 2014. http://american-hearing.org/disorders/congenital-deafness/
Image source: Creative Commons, http://www.alldeaf.com/general-chat/75553-tattoo.html