Q.21 In an experiment, two fly populations are separately maintained for many generations.
Population A contains closely related individuals, whereas Population B contains a set of unrelated individuals.
Over many generations, it was observed that the fitness of individuals in Population A is lower than the fitness of those in Population B, because:

1. Close relatives will not mate with one another.
2. Close relatives compete for common resources more than unrelated individuals.
3. The offspring of close relatives harbour more deleterious recessive mutations.
4. Unrelated individuals are likely to contain new high-fitness genes.

   Correct Answer: C. The offspring of close relatives harbour more deleterious recessive mutations.

   Why Population A Has Lower Fitness

   Population A, with closely related flies, experiences inbreeding over generations. This increases homozygosity, exposing harmful recessive mutations and causing inbreeding depression—reduced fitness from genetic load. Population B, with unrelated flies, promotes outbreeding, masking recessives in heterozygotes and boosting overall fitness through diversity.

   Option Analysis

   Option A: Close relatives will not mate with one another.
   False. In confined Population A, close relatives mate frequently, driving inbreeding—not avoidance.​

   Option B: Close relatives compete for common resources more than unrelated individuals.
   Incorrect. Kin competition exists but does not primarily explain fitness decline; genetic issues dominate.

   Option C: The offspring of close relatives harbour more deleterious recessive mutations.
   Correct. Inbreeding raises homozygote frequency for deleterious recessives, impairing viability, fertility, and survival.

   Option D: Unrelated individuals are likely to contain new high-fitness genes.
   Wrong. Fitness gain in B stems from avoiding inbreeding depression, not guaranteed “new high-fitness genes.”​

   Introduction to Inbreeding Depression Fly Populations

   In an experiment tracking inbreeding depression in fly populations, Population A (closely related individuals) exhibited lower fitness over generations compared to Population B (unrelated flies). This classic setup highlights how mating patterns affect genetic health, crucial for GATE Life Sciences prep on population genetics.​

   Experiment Overview

   Two fly groups were isolated: Population A forced close-kin matings, while Population B allowed random pairings. Fitness—measured by survival, reproduction—dropped in A due to accumulated genetic defects.

   Core Mechanism: Deleterious Recessive Mutations

   Close relatives share alleles, so offspring become homozygous for hidden harmful recessives (e.g., sterility genes). These express phenotypically, slashing fitness via inbreeding depression. Outbreeding in B hides them, preserving vigor.​

   Population	Mating Type	Genetic Effect	Fitness Outcome
   A (Related)	Inbreeding	High homozygosity of deleterious recessives	Lower fitness ​
   B (Unrelated)	Outbreeding	Heterozygosity masks mutations	Higher fitness ​

   Why Other Factors Fail

   • Relatives do mate in isolation (contra A).

   • Resource competition is secondary to genetics (contra B).

   • No evidence mandates “new high-fitness genes” in unrelated flies (contra D).​

   Relevance for GATE Life Sciences

   This question tests Hardy-Weinberg disruptions and inbreeding effects. Focus on recessives’ role in small populations for exams.​