What is the basis for the delay between the atrial and ventricular contractions in a normal heart?

The main responsible for the delay among the atrial and ventricular contractions is the action potential conduction velocity of the AV node cells. If the atria and ventricles contract simultaneously, atria cannot give any help in filling the ventricles, which is needed, especially in those with stiff ventricles as in the elderly. Hence atria have to contract first and then the ventricles.

answer; a male inherits only one allele of the x-linked gene controlling hair color;

i believe the answer is c