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Population Genetics and Evolution

Population Genetics

• Population genetics involves the application of genetic principles to entire populations of organisms

• Population = group of organisms of the same species living in the same geographical area

• Subpopulation = small population unit

• Gene pool = all alleles in population

Population Genetics

• Genotype frequency = proportion of individuals in a population with a specific genotype

• Allele frequency = proportion of alleles in a population

Hardy-Weinberg Principle

• Hardy Weinberg (HW) Principle analyzes the factors which may affect the frequencies of alleles in a population

• HW principle uses a simple equation to calculate allelic frequencies:

P = frequency (f) of all alleles = 1

p = (f) dominant allele

q = (f) recessive allele p + q =1

Hardy Weinberg Principle

HW Principle states that allelic frequencies will remain constant over time if the following conditions are met:

• Mating is random• Allelic frequencies are the same in males

and females• All genotypes have equivalent viability

and fertility

Hardy Weinberg Principle

• Mutation does not occur• Migration into the population is absent• Population is large so that allelic

variations do not occur by chance

These idealized conditions are never met, but HW principle permits analysis of mechanisms responsible for changes in allelic frequencies

Hardy Weinberg Principle

• HW equation can be used to calculate genotype frequencies in a population:

pp = homozygous dominant genotype

qq = recessive genotype

pq = heterozygous genotype

Since p+q = 1, binomial expansion:

p2 + 2pq + q2 = 1

Hardy Weinberg Principle

p2 = pp = frequency of homozygous

dominant genotype

2pq = frequency of heterozygous

genotype

q2 = frequency of recessive genotype• Fixed allele: frequency = 1• Lost allele: frequency = 0• Rare alleles mostly heterozygous

Hardy Weinberg Principle

• Phenotype = genotype = q2 in recessive individuals; q2 is used to calculate the frequencies of the homozygous dominant (p2) and heterozygous individuals (2pq)

• HW equation can be used to determine the frequency of recessive disease alleles in a population and carrier frequency

Hardy-Weinberg Principle

• Hardy-Weinberg frequencies can be extended to multiple alleles:

-- Frequency of any homozygote=square of allele frequency

-- Frequency of any heterozygote = 2 X product of allele frequencies

Hardy-Weinberg Principle

• X-linked genes are a special case because males have only one X-chromosome

• Genotype frequencies among males are the same as allele frequencies:

Frequency of H males = p

Frequency of h males = q

DNA Typing

• DNA typing involves the use of polymorphisms to link individuals with tissue samples

• Highly polymorphic genes are used in DNA typing

• Polymorphic alleles may differ in frequency among subpopulations = population substructure

• DNA exclusions are definitive

Allelic Variation

• Allelic variation may result from differences in the number of units repeated in tandem = simple tandem repeat polymorphism (STRP)

• STRPs can be used to map DNA since they generate RFLPs which can be detected by Southern blot analysis

Genetic Inbreeding

• Inbreeding means mating between relatives

• Inbreeding results in an excess of homozygotes compared with random mating

• In most species, inbreeding is harmful due to rare recessive alleles that would not otherwise become homozygous

Genetics and Evolution

• Evolution refers to changes in the gene pool resulting from mutations which produce phenotypic changes subject to the forces of natural selection

• Natural selection refers to environmental interaction with phenotypic variants to select for those with reproductive advantage

Evolution

Processes which result in the formation of new species include:

• Mutation: origin of new genotypes

• Migration: movement among subpopulations

• Natural selection: results in adaptation

• Random genetic drift: changes in allele frequencies

Natural Selection

• Natural selection is the driving force of adaptive evolution and is a consequence of the hereditary differences among organisms and their ability to survive in the surrounding environment

• Adaptation: progressive genetic improvement in populations due to natural selection

Natural Selection

Natural selection depends on the following principles:

• More organisms are produced than can survive and reproduce

• Organisms differ in their ability to survive and reproduce, based on genotypic differences

• The genotypes that promote survival are favored and are reproduced

Fitness

• Fitness is the relative ability of genotypes to survive and reproduce

• Relative fitness measures the comparative contribution of each parental genotype to the pool of offspring genotypes in each generation

• Selection coefficient refers to selective disadvantage of genotype

Allelic Selection

• Frequency of very common or rare alleles changes very slowly

• Selection for or against very rare recessive alleles is inefficient

• Lethal genotypes are not passed on to the next generation and their frequency decreases over time

• Frequency of favored genotypes increases over time

Selection in Diploids

• Frequency of favored dominant allele changes slowly if allele is common

• Frequency of favored recessive allele changes slowly if the allele is rare

• Rare alleles are found most frequently in heterozygotes and when favored allele is dominant recessive alleles in heterozygotes are not exposed to natural selection

Selection in Diploids

• Selection for or against a recessive allele is very inefficient when the recessive allele is rare

• This accounts for the persistence of rare recessive disease causing alleles in the human population in heterozygote carriers, even if they are lethal when homozygous

Selection in Diploids

• Selection can be balanced by new mutations

• New mutations often generate harmful alleles and prevent their elimination from the population by natural selection

• Some changes in allele frequency are random due to genetic drift among subpopulations

Heterozygote Superiority

• Heterozygote superiority = fitness (measurement of viability and fertility) of heterozygote is greater than both homozygotes

• In sickle cell anemia overdominance of the heterozygote carriers is observed because they are more resistant to malaria

Maternal Inheritance

• Maternal inheritance refers to the transmission of genes only through the female

• In higher animals, mitochondrial DNA shows maternal inheritance

• Mitochondria are maternally inherited because the egg is the major contributor of cytoplasm to the zygote

Maternal Inheritance

• Males do no not transmit mitochondria DNA to offspring

• Some rare genetic disorders are the result of mutations in mitochondrial DNA and are transmitted from mother to all offspring

• Recombination does not occur in mitochondrial DNA making it a good genetic marker for human ancestry

Phylogenetic Tree

• Phylogenetic Tree represents a depiction of the lines of descent connecting mitochondrial DNA inheritance patterns

• mtDNA maternally inherited

• Greatest mtDNA diversity in African populations