 # Population Genetics. Mendelain populations and the gene pool Inheritance and maintenance of alleles and genes within a population of randomly breeding.

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Population Genetics

Mendelain populations and the gene pool Inheritance and maintenance of alleles and genes within a population of randomly breeding individuals Study of how often or frequent genes and/or alleles appear in the population Genotypic frequencies – how often do certain allelic combinations appear Allelic frequencies - how often does an individual allele appear

Genotypic frequencies frequency a particular genotype appears (combination of alleles) for moths at right out of 497 moths collected BB appears 452 times Bb appears 43 times bb appears 2 times Frequencies BB 452 ÷ 492 = 0.909 Bb 43 ÷ 492 = 0.087 bb 2 ÷ 492 = 0.004 Total 1.000 BB Bb bb

What about alleles that do show simple dominant - recessive relationship? How does genotypic frequency really demonstrate flux or change in frequencies of the dominant allele? What if there are multiple alleles? Allelic frequencies

Allelic frequency Allelic frequency = Number of copies of a given allele divided by sum of counts of all alleles BB appears 452 times Bb appears 43 times bb appears 2 times 492 moths 994 alleles Frequencies B (904 + 43) ÷ 994 = 0.953 b (43 + 4) ÷ 994 = 0.047 Total 1.000 BB Bb bb

Can also calculate it from the genotypic frequencies BB was 0.909 Bb was 0.087 bb was 0.004 Therefore frequency of B = Frequency of BB + ½ frequency of Bb f(B) =.909 + ½ 0.087 =.909 +.0435 =.9525 F(b) = 0.004 + ½ 0.087 = 0.004 + 0.0435 = 0.047 What about multiple alleles?

GenotypeNumber A 1 A 1 4 A 1 A 2 41 A 2 A 2 84 A 1 A 3 25 A 2 A 3 88 A 3 A 3 32 Total274 f(A 1 )= Total number of A 1 in population divided by total number of alleles

GenotypeNumber A 1 A 1 4 A 1 A 2 41 A 2 A 2 84 A 1 A 3 25 A 2 A 3 88 A 3 A 3 32 Total274 f(A 1 )= Total number of A 1 in population divided by total number of alleles

GenotypeNumberNumber of A 1 A 1 A 1 42 X 4 A 1 A 2 4141 A 2 A 2 84 A 1 A 3 2525 A 2 A 3 88 A 3 A 3 32 Total274 f(A 1 ) = ((2 X 4) + 41 + 25) ÷ (2 X 274) = (8 +41 + 25) ÷ 548 = 74 ÷ 548 = 0.135

Allelic frequencies at X linked locus same principle However remember for humans that males only have one X So that F(one allele = 2 X the homzygous genotype) + the number of heterozygotes + the males with the phenotype all divided by the number of alleles in the population (2 X females) plus males.

Hardy – Weinberg “law” Frequencies of alleles and genotypes within a population will remain in a particular balance or equilibrium that is described by the equation Consider a monohybrid cross, Aa X Aa Frequency of A in population will be defined as p Frequency of a in population will be defined as q

GametesA (p)a (q) A (p)AA(pp)Aa(pq) a (q) Aa(pq)aa(qq) Frequency of AA offspring is then p 2 Frequency of aa offspring is then q 2 Frequency of Aa offspring then 2pq Frequency of an allele being present is = 1

p 2 + 2pq + q 2 = 1 Where p = frequency of “dominant” allele and q = frequency of “recessive” allele For the moth example (0.9525) 2 + (2 X (0.953 X 0.047)) + (0.047) 2 0.907 + (2 x 0.045) +.002.907 +.09 +.002 =.999 Is this good enough?

Can be extended to more than two alleles Two alleles (p + q) 2 = 1 Three alleles (p + q + r) 2 = 1 And X – linked alleles Can be used to det4ermine frequencies of one allele if the presence of one allele is known

Conditions or assumptions for the Hardy – Weinberg law to be true Infinitely large population (?) Randomly mating population (with respect to trait) No mutation (with respect to locus or trait) No migration (with respect to locus or trait) No natural selection (with respect to locus or trait) Frequencies of alleles do not change over time

Population variation How is it quantitated? Proportion of polymorphic loci Heterozygosity

Population variation in space and time for alleles Blue mussel Cline –systematic variation in allele frequency across geography

Temporal variation

Population variation Variation at many loci How is it detected? PCR Sequencing Protein electrophoresis VNTRs SNTRs Synonymous vs. non-synonymous variations or chnages

How is population variation of loci obtained Random events Mutation Gain and loss of genes from the gene pool Founder effect Bottleneck effect Random genetic drift Selection Migration

Mutations may be lost or fixed within a population

Selection and speciation Selection coefficient Heterozygote superiority

Selection against recessive lethal

Fitness

Problems TextStudy Guide 22.1 - 22.5pg 502 – 505 1-15

Terms Mendelian population Gene pool Genotypic frequencies Hardy-Weinberg law Genetic drift Random mating Cline Random genetic drift

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