6.
In an experiment, two fly populations are separately maintained for many generations.
Population A contains closely-related individuals. Population B contains a set of
unrelated individuals. Over many generations it is observed that the fitness of individuals
in population A is lower than the fitness of those in population B. Why?
a. Close relatives will not mate with one another
b. Close relatives compete for common resources more than unrelated individuals
c. The offspring of close relatives harbour more deleterious recessive mutations
d. Unrelated individuals are likely to contain new high-fitness genes
Inbreeding depression occurs when mating between closely related individuals reduces offspring fitness due to increased homozygosity for harmful recessive alleles. In the experiment, Population A (closely related flies) shows lower fitness than Population B (unrelated flies) over generations. This CSIR NET Life Sciences question tests understanding of genetic consequences in small, inbred populations.
Correct Answer
Option c. The offspring of close relatives harbour more deleterious recessive mutations is correct. In Population A, inbreeding raises homozygosity, unmasking recessive deleterious mutations that lower fitness traits like survival and reproduction. Studies on Drosophila confirm this, with inbred lines showing 24-79% fitness drops from expressed harmful alleles.
Option Analysis
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a. Close relatives will not mate with one another: Incorrect. In small populations like A, close relatives do mate due to limited choices, driving inbreeding rather than avoiding it.
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b. Close relatives compete for common resources more than unrelated individuals: Incorrect. Resource competition affects all populations similarly; reduced fitness in A stems from genetic issues, not ecological ones.
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c. The offspring of close relatives harbour more deleterious recessive mutations: Correct, as explained above. Homozygosity exposes hidden recessives, confirmed in fly experiments over generations.
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d. Unrelated individuals are likely to contain new high-fitness genes: Incorrect. Population B’s advantage comes from masking deleterious alleles via heterozygosity, not introducing superior genes.
Inbreeding Depression Mechanism
Inbreeding increases the inbreeding coefficient (F), elevating homozygosity for recessive lethals or detrimentals. Drosophila studies show inbred males have shorter lifespans (e.g., 0.39 lethal equivalents per gamete) and higher age-specific mortality, supporting mutation-accumulation theory of senescence. Outbreeding in Population B maintains heterozygote advantage, preserving fitness.
CSIR NET Relevance
This question appears in CSIR NET Life Sciences exams, linking population genetics, evolution, and quantitative traits. Key for topics like genetic load and small population effects; practice similar PYQs on Drosophila inbreeding for mastery.
