Flower diameter in sunflowers is a quantitative trait A plan

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

Why is the answer 3/8?

Solution

F1 plant of 18 cm flower diameter is crossed to palnt with 6 cm flower diameter. The diameter of the fower is influenced by 3 genes (6, 18 and 30 cm) since, 6 cm and 30 cm plants crossed have produced offspring with 18 cm. Now, cross between 18 cm and 6 cm produces 2³ or 8 gametes. Therefore, the probability of an offspring with one additive allele is 3/8. Option D is correct.