If they are both heterozygous, it means they both have a Dd genotype. If you draw out a punnet square Dd x Dd you end up with the following possible genotypes for the puppy:
DD, Dd, Dd, dd
In order for the puppy to be deaf, it must be dd since deafness is recessive, which is 1/4 (25%)
The best-known neurotransmitters responsible for such fast, but short-lived excitatory action are acetylcholine, norepinephrine, and epinephrine while GABA is the major inhibitory neurotransmitter. Neurons are the basic building blocks of the nervous system.
