Population Genetics

Population Genetics

Which of the following increases genetic variation?

Genetic variation arises via mechanisms generating new alleles, new gene combinations, or new gene copies. Gene duplication creates extra copy of existing gene free to accumulate mutations and acquire novel functions, expanding gene families and proteome diversity, classic example globin genes. Mutation in mitochondrial DNA alters cytoplasmic variation but not nuclear diversity greatly. Differential expression based on parental imprinting changes phenotype not genotype. Loss of chromosome causes aneuploidy and usually reduces viable variation. Among listed options, duplication is well recognized major source of evolutionary innovation and increased heritable genomic variability across taxa.

Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 14: Gene Duplication as Source of Variation

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Population Genetics

Hardy–Weinberg principle applies only to

Hardy-Weinberg model assumes diploid organisms reproducing sexually with random fusion of male and female gametes, creating genotype frequencies from allele frequencies via binomial expansion (p plus q) squared. Haploid or clonal asexual populations lack formation of diploid genotypes from two independent gamete pools, so p squared, two pq, q squared framework does not apply appropriately. While allele frequencies in haploids still evolve via drift and selection, genotype equilibrium concept requires diploid sexual shuffling each generation. Thus principle is formulated specifically for diploid, sexually reproducing populations with Mendelian segregation.

Ref: Hartl & Clark, Principles of Population Genetics, Chapter 2: Scope of Hardy-Weinberg Model

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Population Genetics

In a population, frequency of AA = 0.25 and Aa = 0.5. Frequency of allele A is

Allele frequency can be derived accurately from genotype frequencies using gene counting method. Each AA individual contributes two A alleles, heterozygote contributes one of each type. Therefore p equals frequency of AA plus half frequency of Aa. With AA equals 0.25 and Aa equals 0.5, p equals 0.25 plus 0.25 equals 0.5, and q also 0.5. When genotype frequencies already follow Hardy-Weinberg expectation, this estimate coincides with square root of AA. Such estimation is fundamental for population screening and for testing whether observed counts deviate from equilibrium proportions within studied cohort.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 20: Estimating Allele Frequencies Method

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Population Genetics

If p² = 0.64, value of p is

Homozygous dominant genotype frequency equals p squared under Hardy-Weinberg equilibrium expectation. Given p squared equals 0.64, p corresponds to square root of genotype frequency, since squaring allele frequency yields genotype proportion. Square root of 0.64 equals 0.8, implying allele A exists at eighty percent frequency in population. Consequently q equals 0.2, giving heterozygotes at 0.32 and recessive homozygotes at 0.04 after expansion. This direct conversion illustrates relationship between observed genotype counts and underlying allele frequencies, essential when dominant phenotype lumps AA and Aa indistinguishably.

Ref: Hartl & Ruvolo, Genetics, 9th ed., Chapter 19: From Genotype Frequency to Allele Frequency

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Population Genetics

If p = 0.7, q is equal to

For two-allele system at single locus, sum of allele frequencies equals one, representing entire allelic pool, expressed as p plus q equals one. If p denotes frequency of allele A at 0.7, then q equals one minus p by mathematical complementarity. Calculation yields one minus 0.7 equals 0.3, frequency of alternative allele a. This relationship underlies all Hardy-Weinberg calculations, allowing determination of q when p known and vice versa. It holds regardless of genotype distribution or population structure and forms basis for estimating heterozygote and recessive homozygote frequencies via binomial expansion.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 20: Allele Frequency Summation Principle

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Population Genetics

Which condition violates Hardy–Weinberg equilibrium?

Hardy-Weinberg equilibrium assumes infinitely large population size so sampling error becomes negligible across generations. Small population size violates this assumption, causing genetic drift where allele frequencies fluctuate randomly each generation due to limited gamete sampling forming zygotes. Drift leads to random loss or fixation of alleles, increased homozygosity, and deviation from p squared, two pq, q squared expectations. Thus small size acts as potent evolutionary force introducing stochasticity, unlike random mating or absence of mutation which are required conditions for stability at equilibrium.

Ref: NCBI Scitable, Nature Education, Hardy-Weinberg Equilibrium Conditions and Small Population Effects

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Population Genetics

Assortative mating leads to

Assortative mating occurs when individuals choose mates phenotypically similar to themselves for particular traits or genetically similar background. Positive assortative mating increases mating between similar genotypes, resembling inbreeding effect, elevating frequency of homozygous genotypes and reducing observed heterozygotes compared with random expectation. Negative assortative mating does opposite, favoring dissimilar partners and increasing heterozygosity. Unlike disassortative mating that promotes diversity, positive assortment for a locus mimics autozygosity effect, raising both dominant and recessive homozygotes while allele frequencies themselves remain unchanged by mating system alone.

Ref: Hartl & Clark, Principles of Population Genetics, Chapter 5: Assortative Mating Consequences

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Population Genetics

Non-random mating affects mainly

Random mating ensures genotype frequencies follow Hardy-Weinberg proportions p squared, two pq, q squared derived solely from allele frequencies. Nonrandom mating, including inbreeding or positive assortative mating based on phenotype, alters probability that similar genotypes pair, shifting genotype proportions while allele frequencies remain fundamentally unchanged in absence of other forces. For instance, consanguinity increases homozygosity at expense of heterozygosity. Therefore nonrandom mating primarily impacts genotype frequencies, causing apparent deviation from Hardy-Weinberg expectations, but does not systematically change p and q unless coupled with selection or drift.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 20: Nonrandom Mating Effects on Genotypes

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Population Genetics

Gene flow refers to

Gene flow describes exchange of alleles between populations through migration of individuals followed by interbreeding. Migrants carry allele frequencies characteristic of source population, and their gametes introduce variants into recipient gene pool, directly altering p and q locally. Effective migration reduces genetic divergence, homogenizes populations, can introduce novel variation or spread advantageous alleles across ranges. It opposes genetic drift and selection-driven differentiation that create divergence. Unlike drift which removes variation randomly, gene flow moves existing alleles spatially and is measured quantitatively by migration rate m.

Ref: NCBI Bookshelf, Hartl Population Genetics: Migration, Gene Flow, and Allele Exchange

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Population Genetics

Sickle cell anemia persistence in population is due to

Sickle cell allele causes severe hemolytic disease in homozygous recessive state but confers malaria resistance in heterozygous condition in regions endemic for Plasmodium falciparum. Heterozygotes survive better than both AA individuals susceptible to malaria and SS patients with severe anemia, exemplifying overdominance with highest fitness in heterozygote. This balancing selection maintains S allele at frequency far higher than mutation alone would predict, despite strong purifying selection against SS. Heterozygote advantage therefore explains persistence of otherwise deleterious allele in African, Mediterranean, and Indian populations historically exposed.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 20: Hemoglobin Polymorphism and Balancing Selection

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Population Genetics

Heterozygote advantage is an example of

Balancing selection maintains multiple alleles stably in population rather than favoring single variant toward fixation and loss. Heterozygote advantage, also called overdominance, is a classic form of balancing selection where heterozygous genotype fitness exceeds both homozygous genotypes, preventing elimination of either allele. This opposes directional and disruptive selection that reduce variation over time. Classic examples include sickle cell polymorphism and MHC diversity maintained by pathogen pressure. By selecting against both homozygote classes, heterozygote superiority preserves stable polymorphism, categorizing it under balancing selection mechanisms counteracting genetic drift.

Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 21: Balanced Polymorphism and Overdominance Theory

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Population Genetics

Inbreeding leads to

Inbreeding denotes mating between relatives sharing alleles identical by descent from common ancestor, increasing probability offspring receives same ancestral allele from both parents via two lineages. This elevates autozygosity, converting heterozygotes into homozygotes without altering allele frequencies in population. Inbreeding coefficient F quantifies fractional reduction in heterozygosity relative to random mating expectation, with genotype frequencies p squared plus pqF, two pq times one minus F, q squared plus pqF. Consequences include increased expression of recessive deleterious alleles and inbreeding depression affecting fitness.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 20: Inbreeding and Autozygosity

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