Question

The objections to the 'Dominance Hypothesis' of heterosis and inbreeding depression are-
(A). Mostly, it is not possible to isolate inbreds as vigorous as hybrids
(B). Asymmetrical distribution of quantitative traits in the F2 population
(C). Magnitude of heterosis in inbred lines has increased over time, which was not expected as per the hypothesis
(D). Faster inbreeding depression in tetraploids
Choose the correct answer from the options given below:

• A. (A), (B) and (C) only.
• B. (A), (B) and (D) only.
• C. (A), (C) and (D) only.
• D. (B), (C) and (D) only.

Hint:

Remember the two classic objections to the dominance hypothesis of heterosis:
- Failure to isolate inbreds as vigorous as the \(F_1\) hybrid.
- Symmetrical (not asymmetrical) distribution of quantitative traits in the \(F_2\) generation.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Heterosis (hybrid vigor) describes the increased performance of an \(F_1\) hybrid compared to its inbred parents.
The dominance hypothesis suggests that heterosis is caused by the masking of deleterious recessive alleles from one parent by dominant alleles from the other parent.

Step 2: Detailed Explanation:
While the dominance hypothesis is a major model for heterosis, early geneticists raised several objections to it:
(A) is a valid objection because if heterosis is due to dominant alleles, it should be theoretically possible to isolate homozygous dominant lines (e.g., \(AABBCC\)) that are as vigorous as the heterozygous \(F_1\) hybrid.
In practice, breeders have not been able to isolate inbred lines with vigor equal to the hybrid.
(B) is a valid objection because segregating dominant and recessive alleles in the \(F_2\) generation should produce an asymmetrical (skewed) distribution of quantitative traits based on the 3:1 ratio.
In practice, \(F_2\) populations typically display a symmetrical, normal bell curve distribution.
(C) is a valid objection because the gradual accumulation of dominant alleles in inbred lines over generations of selection was expected to decrease the relative magnitude of heterosis when crossed, which is inconsistent with observed breeding results.
(D) is incorrect because tetraploid plants show a slower rate of inbreeding depression than diploid plants.
This is due to their complex chromosome segregation, which requires more generations of selfing to reach complete homozygosity.
Therefore, (A), (B), and (C) are the correct objections to the dominance hypothesis.

Step 3: Final Answer:
The correct combination is (A), (B), and (C) only, corresponding to option (A).