Genetic Variation: Chapter 18

• Genetic variation in populations is related to Chapter 18

Mutation

• Mutation presents itself as an important attribute of populations
• New mutations will rarely improve existing genes unless their effect is neutral
• Environmental changes invoke genetic responses based on available genetic variation
• Insect populations rapidly change genetically when exposed to pesticides such as dieldrin and DDT

Adaptation to DDT

• Insects can resist DDT through multiple adaptive mechanisms
• These mechanisms involve increased lipid content
• As well as enzymes that breakdown DDT
• Reduced nervous toxic response is another
• Changes in cuticle permeability are another method
• Behavioral changes, such as contact reduction with DDT, also help

Random Mutations

• Pre-adaptive mutations occur randomly before exposure to a selective agent
• Post-adaptive mutations are non-random and occur after exposure

Replica-Plating

• Replica plating was performed by J. & E. Lederberg
• In replica-plating, colonies on a master plate (without phage) are transferred to replica plates with phage medium using a velvet-covered block

Genetic Polymorphism

• Genetic polymorphism can result from mutations
• Mutations may be harmful, beneficial, neutral or recessive
• Genetic polymorphism results from the accumulation of mutations
• Genetic polymorphism also results from inversions, translocation (duplications), and extra chromosomes

Neo-Darwinism and Genetic Polymorphism

• Neutral or deleterious recessive mutations create a reservoir of genetic variation
• Genetic polymorphism is a greater source of genetic variation than new mutations in each generation

Genetic Polymorphisms Example

• Drosophila pseudoobscura fruit flies show genetic polymorphisms
• Populations in various locations are polymorphic for a variety of gene arrangements.

Continuous Variation

• Continuous variation can be caused by polygenes, where several genes each contribute a small phenotypic effect
• Quantitative Trait Loci (QTL) also contribute
• Independent assortment plays a role
• Results in a normal distribution

Quantitative Trait Loci

• Quantitative Trait Loci are all the genes (alleles) in a chromosome that affect quantitative aspects of phenotype i.e size or shape
• Tail growth and body weight in some rats can be attributed to 4 QTLs
• Environmental effects quantitatively influence some genes
• Heritability describes how much genetic differences affect traits and selection

Continuous Variation Example

• Small heritable changes lead to variation, which leads to natural selection
• Body size distribution can be an example of continuous variation in a population

Rabbit Color Patterns and Sticklebacks

• Rabbit color patterns are due to 3 gene pairs, each with 2 alleles (1 white, 1 black)
• Three-spine sticklebacks exhibit variations in dermal plates and pelvic girdle structures

Basic Concepts

• Gene frequency (allele frequency) is the proportion of an allele in a population
• Total gene frequency for all alleles equals 1
• Gene pool is the sum of all genes in reproductive gametes in a population

Gene Frequencies

• Consider two alleles: T (dominant, tasting) and t (recessive, non-tasting)
• In a population of 200 individuals, there are: 90 TT (tasters), 60 Tt (tasters), and 50 tt (non-tasters)
• At this diploid locus, there are 400 genes
• The frequency of T is (90 x 2 + 60) / 400 = 0.6
• The frequency of t is (100 + 60) / 400 = 0.4

Gene Frequency Calculation Example

• Given frequencies of genotypes, allele frequencies can be derived:
• T = 0.45 (TT) + 1/2 * (0.30 Tt) = 0.45 + 0.15 = 0.60
• t = 0.25 (tt) + 1/2 * (0.30 Tt) = 0.25 + 0.15 = 0.40

Genotype frequencies

• If p represents the frequency of allele T and q represents the frequency of allele t
• p + q = 1
• Genotype frequencies are p² (TT) + 2pq (Tt) + q² (tt)
• (p + q)² = p² + 2pq + q² based on Binomial expansion

Equilibrium

• Equilibrium with 3 alleles
• If 3 alleles exist on the same locus, the trinomial expansion applies: (p + q + r)² = p² + 2pq + 2pr + q² + 2qr + r²

Hardy-Weinberg Equilibrium

• Allele frequencies reach equilibrium under specific conditions
• Frequencies become stable from one generation to the next
• aka Conservation of gene frequencies
• Conditions include: Panmixia (random mating), equal viability of all genotypes, and a static population
• These conditions are rarely fully met in nature

Hardy-Weinberg principle

• States that in a randomly mating population, allele frequencies remain constant, and equilibrium will present itself, unless external forces influence them.

Real-World Examples

• Some natural populations have reached Hardy-Weinberg equilibrium.
• Random mating is demonstrated in some populations.

Alleles at Two or More Loci

• Considering frequencies p, q, r, s
• Expansion: (pr + ps + qr + qs)²
• Complicated by linkage between two loci.
• With gametes in repulsion (recombination): Ab, aB
• Gametes in coupling (non-recombination): AB, ab.

Linkage Equilibrium

• Indicates that the frequencies of various gametes align with expectations
• It is called linkage disequilibrium if expectations are not met
• Lower recombination frequency is a tell

Disequilibrium

• Combinations of several alleles at various loci leading to higher reproductive success indicates disequilibrium
• Linkage between genes can be preserved over time with restricted recombination demonstrating disequilibrium
• This results in coadapted gene complexes

Sex Linkage

• In females (XX), frequencies match autosomal genes
• In males, equilibrium may be reached after several generations
• Frequencies are equal in both sexes at equilibrium, but genotypes differ

Natural Populations

• In some populations, dominant-recessive situations lead to a similar phenotype
• Carrier heterozygotes are relatively frequent when recessive phenotypes (aa) are rare

Inbreeding

• Hardy-Weinberg equilibrium is not maintained during non-random mating
• Inbreeding is mating within a very small group
• Self-fertilization is an extreme case of inbreeding
• Inbreeding quantified by inbreeding coefficient, F
• F represents the likelihood that two alleles in a diploid zygote are identical
• The range is from 1 (complete homozygosity) to 0 (complete heterozygosity)
• Expected frequencies from Hardy-Weinberg might be modified when F=1
• Inbreeding depression is when rare deleterious alleles appear in inbred populations