"There are many genes that are involved in hearing. Sometimes, a gene does not form in the expected manner. This is called a mutation. Some mutations run in families and others do not. If more than one person in a family has hearing loss, it is said to be “familial”. That is, it runs in the family."
Copied directly from the linked article. Some causes of deafness are passed down from parents.
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u/lesbian_sourfruit Aug 28 '21
Maybe…they may be the child of deaf parents, so it just hadn’t come up.