Multiple cross were made between true-breeding lines of black and yellow Labrador retrievers. All the F₁ progeny were black. When these progeny were intercrossed, they produced an F₂ consisting of 91 black, 39 yellow and 30 chocolate.

(a) Propose an explanation for the inheritance of coat colour Labrador retrievers.

(b) Propose a biochemical pathway for a coat colour determination and indicate how the relevant genes control coat colouration.