Young: Medical Genetics 
Multiple Choice QuestionsYoung: Medical Genetics
You should now have constructed a pedigree similar to the one shown above. The key point in the family history is that the parents of the affected child, related through their grandparents, are second cousins. Although this is a relatively distant relationship, and second cousins would normally be quoted an additional risk of less than 1% for having a child with an autosomal recessive disorder, this parental consanguinity raises the possibility that the daughter has an autosomal recessive form of hearing loss. It is known that at least 50% of unexplained severe sensorineural hearing loss in children has a genetic basis, most commonly showing autosomal recessive inheritance. Mutations in a gap junction gene known as connexion-26 are the commonest cause of severe congenital sensorineural hearing loss in the European population (page 168).
In a family such as this, investigations would be undertaken to try to determine the cause of the child’s hearing loss. These would almost certainly include connexion-26 mutation analysis. Unless a specific environmental cause is identified, the parents would be counselled on the basis of a probable recurrence risk of 25% and alerted to the importance of ensuring that hearing assessment be undertaken on their two sons. Although genetic hearing loss generally presents at a similar age in children, it is recognised that occasionally there can be some variation in age of onset. However, this reported variation in age of onset is unlikely to account for the late onset hearing loss in the maternal grandfather. This is much more likely to be attributable to a common condition such as otosclerosis or other factors such as longstanding occupational exposure to loud noise.
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