Q.45 In a relatively large but finite and closed population of sexually
reproducing diploid organisms, the frequency of homozygous genotype
PP changes from 0.40 to 0.50 and that of pp changes from 0.40
to 0.41 in a span of 10 generations. Which of the following is the most
likely cause for the above change in frequency of the PP genotype?
- Non-random mating
- Random genetic drift
- Selection
- Combination of non-random mating and random genetic drift
Population Genetics: Why PP Genotype Frequency Increases from 0.40 to 0.50
In a closed diploid population, the homozygous PP genotype rises significantly from 0.40 to 0.50 over 10 generations, while pp edges up slightly from 0.40 to 0.41. Selection emerges as the most likely cause due to the directional shift favoring PP.
Correct Answer: Selection
Selection best explains the disproportionate increase in PP (Δ=0.10) compared to pp (Δ=0.01). This indicates PP confers higher fitness, shifting both genotype and underlying allele frequencies directionally over 10 generations in a relatively large population.
Natural selection favors PP individuals, leading to their greater survival or reproduction. Initial frequencies suggest p ≈ q ≈ 0.632 (since PP + pp ≈ 0.80 implies heterozygotes ≈ 0.20), but PP’s rapid gain implies positive selection on P allele, violating Hardy-Weinberg by changing allele ratios.
Non-Random Mating Explained
Non-random mating, like inbreeding or assortative mating, alters genotype frequencies by increasing homozygotes (both PP and pp) while keeping allele frequencies stable.
Here, PP surges far more than pp, which rules it out—equal homozygote boosts would occur without allele shifts. It disrupts Hardy-Weinberg but cannot produce such asymmetric change alone.
Random Genetic Drift Effects
Genetic drift causes random allele frequency fluctuations, more pronounced in small populations; in large ones, changes are minimal over 10 generations.
The consistent, substantial PP increase (not random zigzags) and slight pp rise contradict drift, which lacks directionality and symmetry in effect across genotypes.
Why Not Combined Forces?
A mix of non-random mating and drift might elevate both homozygotes modestly but fails to justify PP’s outsized gain (10x pp’s change). Selection provides the clear differential fitness signal absent in combinations.
Key Takeaway for GATE Biotech
Primary Keyword Focus: In finite closed populations, selection uniquely drives unequal homozygote shifts. Use H-W deviations to diagnose: genotype-only (mating), allele-random (drift), allele-directed (selection).
