Flower Diameter in Sunflowers Is a Quantitative Trait

Flower diameter in sunflowers is a quantitative trait. A plant with 6-cm flowers, from a highly inbred strain, is crossed to a plant with 30-cm flowers, also from a highly inbred strain. The F₁ have 18-cm flowers. F₁ × F₁ crosses yield F₂ plants with flowers ranging from 6 to 30 cm in diameter, in approximately 4-cm intervals (6, 10, 14, 18, 22, 26, 30) . An 18-cm F₁ plant is crossed to a 6-cm plant. What is the probability of an offspring with one additive allele, if all genes that influence this trait are unlinked?

A) 1/3
B) 1/4
C) 1/6
D) 3/8
E) 1/16