Essentials of Genetics 9th Edition by Michael Palladino William Klug, Michael Cummings, Charlotte Spencer

Exercise 3

Researching their family histories, a deaf couple learns that each of them has relatives through several generations who are deaf. They also learn that one form of deafness can be inherited as an autosomal recessive trait. They plan to have children, and based on the above information, they assume that some or all of their children may be deaf. To their surprise, their first child has normal hearing. The couple turns to you as a geneticist to help explain this situation.
What conclusions could be drawn if their first child had been deaf?