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