Observing the pattern of inheritance one could say that the trait is recessive and autosomal. Since parents did not express the trait, but the children do, the trait is said to be recessive in condition. Since the trait is neither X-linked, nor Y-linked, it is autosomal. Observing the third generation, makes us to understand that there are two different genes. These two genes when become defective, results in deaf-mutism.
The genotypes of the persons in generations I and II are as follows:
o I-1 and I-2 ------------ A/a; B/B
o I-3 and I-4 ------------ A/A; B/b
o II- (1,3,4,5,6) ------------ A/-; B/B
o II-(9,10,12,13,14,15) ----- A/A; B/-
o II-2 and II-7 ------------ a/a; B/B
o I-8 and II-11 ------------ A/A; b/b
Complementation is observed to be involved in generation III and all the progeny have the genotypes A/a; B/b. Hence, there are only normal persons in generation III.