32. In a large wild flower population, assume that no new mutations occur and that no natural selection operates. What factor(s) will affect the frequency of a genotype in this population?
(A) Non-random mating
(B) Gene flow
(C) Out-breeding within the population
(D) Invasion of a new pathogen that kills a large number of individuals in the population
Hardy-Weinberg Equilibrium: Factors Affecting Genotype Frequency in a Population
Introduction
The Hardy-Weinberg equilibrium is one of the cornerstones of population genetics. It provides a mathematical model describing how allele frequencies and genotype frequencies remain constant from one generation to the next in an ideal population. This equilibrium is maintained only when certain assumptions are satisfied, including random mating, no mutation, no migration (gene flow), no natural selection, an infinitely large population, and the absence of genetic drift.
Whenever one or more of these assumptions are violated, genotype frequencies, allele frequencies, or both may change. Understanding which evolutionary forces alter genotype frequencies and which alter allele frequencies is essential for interpreting population genetics problems.
Correct Answer
Correct Answer: (A) and (B)
Detailed Explanation
The question specifies that the population is large, no new mutations occur, and natural selection is absent. Therefore, only the remaining evolutionary forces should be considered.
Genotype frequencies are influenced whenever the assumptions of Hardy-Weinberg equilibrium are violated. Among the given options, non-random mating changes the distribution of genotypes by increasing or decreasing the proportion of homozygous and heterozygous individuals, while gene flow introduces alleles from other populations and therefore changes both allele frequencies and genotype frequencies.
Explanation of Option (A): Non-random Mating
This option is correct.
Hardy-Weinberg equilibrium assumes that mating occurs randomly. If individuals preferentially mate with genetically similar individuals (positive assortative mating or inbreeding) or genetically different individuals (negative assortative mating), genotype frequencies change.
Non-random mating generally increases or decreases the proportion of heterozygotes and homozygotes without necessarily changing allele frequencies immediately. Since genotype frequencies are directly affected, this option is correct.
Explanation of Option (B): Gene Flow
This option is correct.
Gene flow, also called migration, occurs when individuals or gametes move between populations. Migrating individuals introduce new alleles into the population or remove existing alleles, thereby altering allele frequencies.
Once allele frequencies change, genotype frequencies automatically change according to the Hardy-Weinberg equation. Therefore, gene flow directly affects genotype frequencies.
Explanation of Option (C): Out-breeding Within the Population
This option is incorrect.
Out-breeding refers to mating between genetically unrelated individuals within the same population. Such mating is essentially a form of random mating and is one of the assumptions required for Hardy-Weinberg equilibrium.
Since the population is already assumed to be large and randomly mating, ordinary out-breeding within the population does not disturb genotype frequencies beyond normal Hardy-Weinberg expectations.
Explanation of Option (D): Invasion of a New Pathogen That Kills Many Individuals
This option is incorrect under the conditions stated in the question.
If a pathogen kills individuals randomly, irrespective of genotype, the genotype frequencies remain unchanged. If the pathogen selectively eliminates particular genotypes, natural selection would occur. However, the question explicitly states that natural selection does not operate. Therefore, this option cannot be considered a factor affecting genotype frequency under the given assumptions.
Summary of Each Option
| Option | Correct/Incorrect | Reason |
|---|---|---|
| (A) Non-random mating | Correct | Changes genotype frequencies by altering homozygosity and heterozygosity. |
| (B) Gene flow | Correct | Introduces or removes alleles, changing both allele and genotype frequencies. |
| (C) Out-breeding within the population | Incorrect | Maintains random mating and does not disturb Hardy-Weinberg equilibrium. |
| (D) Pathogen invasion | Incorrect | The question assumes natural selection does not operate. |
Hardy-Weinberg Assumptions
| Assumption | Importance |
|---|---|
| Random mating | Maintains expected genotype frequencies. |
| No mutation | Prevents formation of new alleles. |
| No migration (gene flow) | Maintains constant allele frequencies. |
| No natural selection | Ensures equal reproductive success of all genotypes. |
| Large population size | Minimizes genetic drift. |
Factors That Disturb Hardy-Weinberg Equilibrium
| Evolutionary Force | Effect on Population |
|---|---|
| Mutation | Creates new alleles. |
| Gene Flow | Changes allele frequencies through migration. |
| Natural Selection | Favours certain genotypes. |
| Genetic Drift | Random changes in allele frequencies in small populations. |
| Non-random Mating | Alters genotype frequencies. |
Biological Significance
Studying deviations from Hardy-Weinberg equilibrium allows geneticists to identify the evolutionary forces acting on natural populations. Non-random mating influences the distribution of genotypes, while gene flow introduces genetic variation between populations. These mechanisms play important roles in evolution, adaptation, conservation biology, plant breeding, animal breeding, and human population genetics.
Final Answer
Among the given options, the factors that affect genotype frequencies are:
(A) Non-random mating
(B) Gene flow
Correct Answer: (A) and (B)
