14. Two pure line of corn have mean cob length of 9 cm and 3 cm, respectively. The polygenes involved in this trait exhibit additive gene action. Crossing these two lines is expected to produce a progeny population with mean cob length of
(1) 12.0 cm       (2) 7.5 cm
(3) 6.0 cm         (4) 2.75 cm

The expected mean cob length in the progeny from crossing two pure lines of corn (with mean lengths of 9 cm and 3 cm) exhibiting additive gene action in polygenes will be 6.0 cm. This result comes from calculating the simple average of the parental means, as additive polygenic traits typically show this pattern.

Introduction

Quantitative traits like cob length in corn are commonly controlled by multiple genes acting additively. In plant breeding and genetics, understanding how these polygenes influence phenotype enables accurate prediction of progeny traits. When two pure lines differing in cob length are crossed, the additive gene action leads to an offspring population with a mean value intermediate to the parents. This article explains the process using a step-by-step approach and evaluates every possible option provided.

Detailed Explanation and Option Analysis

Understanding Additive Gene Action

• Additive gene action refers to a scenario where the effects of multiple genes sum up to determine the phenotype. For quantitative traits like cob length, each contributing gene increases or decreases the trait by a small amount, leading to continuous variation across populations.

• When pure lines are crossed, all offspring inherit half the alleles from each parent, so their phenotype will closely match the average of the two.

Calculation for Progeny Mean Cob Length

• Parental means are: 9 cm and 3 cm.

• Offspring mean = (Parent 1 mean + Parent 2 mean) / 2 = (9 + 3) / 2 = 6.0 cm.

• This matches the typical result for additive polygenic traits like cob length in corn.

Option-by-Option Explanation

• Option (1) 12.0 cm: This is higher than both parents and could occur only if dominance or epistasis was involved, not additive action. Additive gene action cannot push the trait beyond the parental extremes.

• Option (2) 7.5 cm: This value is not the mean of 9 and 3, and may arise from miscalculation or misunderstanding of genetic influence.

• Option (3) 6.0 cm: CORRECT. Represents the average and is consistent with additive polygenic inheritance for quantitative traits.

• Option (4) 2.75 cm: This value is lower than both parents, which is genetically implausible under additive action unless environmental factors or epistatic interactions are considered. Not correct for this scenario.

Key Points to Remember

• Additive gene action in polygenic traits produces offspring with mean phenotype at the average value of both parents.

• Crossing two pure lines with additive traits is a fundamental principle in plant breeding studies for predicting trait outcomes.

This robust calculation is essential in genetics for understanding, teaching, and applying principles of quantitative inheritance in crops and other organisms.