Question

Given below are two statements: Statement (I): Genes, in general contribute more variance at low frequencies. Statement (II): Additive genetic variance occurs due to genes which show an additive effect on the quantitative trait. In light of the above statements, choose the most appropriate answer from the options given below:

• A. Both Statement (I) and Statement (II) are correct.
• B. Both Statement (I) and Statement (II) are incorrect.
• C. Statement (I) is correct but Statement (II) is incorrect.
• D. Statement (I) is incorrect but Statement (II) is correct.

Hint:

Genetic variance is always maximized at intermediate allele frequencies (\( p = q = 0.5 \)) because the product \( pq \) is maximized.
At extreme frequencies (very low or very high), variance decreases to near zero.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Genetic variance in a population is determined by the gene effects and the allele frequencies at each locus.
For a locus with two alleles with frequencies \( p \) and \( q \), the additive genetic variance is mathematically proportional to the product \( pq \).

Step 2: Detailed Explanation:
Let us evaluate both statements.
- Statement (I) discusses variance at low allele frequencies.
The additive genetic variance contributed by a locus is given by:
\[ V_A = 2pq\alpha^2 \]
where \( \alpha \) is the average effect of gene substitution.
The product \( pq = p(1 - p) \) reaches its maximum value of \( 0.25 \) when \( p = 0.5 \).
At low allele frequencies (where \( p \) or \( q \) approaches 0), the product \( pq \) approaches 0, and the genetic variance becomes very small.
Therefore, genes contribute the least variance at low frequencies, not the most.
Thus, Statement (I) is incorrect.
- Statement (II) defines additive genetic variance.
Additive genetic variance (\( V_A \)) is the variance of breeding values.
It represents the portion of genetic variance that is due to the additive, cumulative effects of individual genes.
This is the component of genetic variance that is directly transmissible to offspring.
Thus, Statement (II) is correct.
Therefore, Statement (I) is incorrect and Statement (II) is correct.

Step 3: Final Answer:
Statement (I) is incorrect but Statement (II) is correct, corresponding to option 4.