The height of the seed head in wheat at maturity is determined by several genes. In one variety, the head is just 9 inches above the ground; in another, it is 33 inches above the ground. Plants from the 9-inch variety were crossed to plants from the 33-inch variety. Among the F₁, the seed head was 21 inches above the ground. After self-fertilization, the F₁ plants produced an F₂ population in which 9-inch and 33-inch plants each appeared with a frequency of 1/256. 

(a) How many genes are involved in the determination of seed head height in these strains of wheat? 

(b) How much does each allele of these genes contribute to seed head height? 

(c) If a 21-inch F₁ plant were crossed to a 9-inch plant, how often would you expect 18-inch wheat to occur in the progeny?