In population genetics, the fate of an allele depends on how it affects the fitness of organisms. When a particular genotype—such as the heterozygote (Aa)—is lethal, it means individuals with this genotype do not survive or reproduce. This scenario has a profound effect on the frequencies of the alleles involved.

The Scenario

• Two alleles at a locus: A and a

• Frequency of a: 0.3

• Genotype Aa is lethal: Only AA and aa survive and reproduce

What Happens Over Generations?

1. Immediate Effect of Lethal Heterozygotes

When Aa individuals die before reproducing, only AA and aa genotypes contribute to the next generation. This is a strong form of selection against the heterozygote.

2. Allele Frequency Dynamics

• Each generation, new Aa individuals are produced by mating between AA and aa, but these die off and do not pass on their genes.

• The only way for allele a to persist is through aa homozygotes.

• However, every time an A and a allele meet in a zygote (forming Aa), that individual is removed from the gene pool.

3. Long-Term Outcome: Loss of One Allele

Over many generations, this selection regime leads to a phenomenon called disruptive selection. The population cannot maintain both alleles because the heterozygote is always eliminated. Eventually, one of the alleles (either A or a) will be lost from the population due to random chance (genetic drift) or initial frequency advantage.

• If allele a starts at a lower frequency (as in this case, 0.3), it is much more likely to be lost.

• The population will become fixed for one allele—usually the one that was more common to begin with.

4. Why Not 1:1 or Stable Frequencies?

The system cannot maintain a stable polymorphism because the heterozygote is always removed. Only homozygotes survive, and random mating will continue to produce lethal heterozygotes, reducing the frequency of the rarer allele each generation.

Which Option Is Correct?

• (1) Allele frequency will be 1:1
  Incorrect: This would require both alleles to be maintained, which is impossible when heterozygotes are lethal.

• (2) Allele frequency will remain same to that of present
  Incorrect: Selection against heterozygotes will change allele frequencies each generation.

• (3) Allele ‘a’ would be lost from population
  Correct: The rarer allele (a) will be eliminated over generations because it cannot be maintained when all Aa die.

• (4) Population will disrupt into two new species
  Incorrect: Speciation requires more than just the loss of a genotype; it involves reproductive isolation and other factors.

The Science Behind the Answer

Natural selection eliminates the heterozygote genotype each generation. The rarer allele, in this case ‘a’, has fewer chances to persist because every time it pairs with ‘A’, the resulting individual does not survive. Over time, this leads to the loss of allele ‘a’ from the population.

Correct answer: (3) Allele ‘a’ would be lost from population