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BIOLOGY 2C03 (150)
Joe Kim (16)
Lecture 2

Lecture 2 – January 15, 2013.docx

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McMaster University
Joe Kim

BIO 2C03 2013 Lecture 2 – January 15, 2013  Mendelian Population – an interbreeding group that share the same “gene pool”  Evolution – changes in “gene pool”; changes in the frequencies of different alleles Population Genetics of β-globin alleles  Observation: The β mutation appears with high frequency in several contemporary populations  Hypothesis: The β mutation confers a selective advantage  β is highest in areas where malaria is prevalent  General Symptoms of Malaria o Headache o Nausea o High fever o Vomiting o Flu-like symptoms o Further serious complication involving the kidneys and brain can then develop leading to delirium and coma o Malarial infections are chronic and individuals are susceptible to other infections which may lead to death  Malaria is caused by a protozoan such as Plasmodium falciparum  Carried by a vector, in this case the mosquito, but begins larval development in the mammalian host, transferred through a mosquito bite  Plasmodium larvae mature in the liver of the host animal and later in the red blood cells S S S A A A  Individuals with β β genotype and β β are more resistant to malaria than those with β β genotype o Sickled red blood cells are fragile and the average RBC lifespan in heterozygotes is shorter due to the abnormal haemoglobin o Shorter RBC lifespan interrupts the life cycle of the Plasmodium larvae, preventing the proliferation of malaria in the human host  Natural selection maintains both alleles in population – both homozygous phenotypes are maintained = balanced polymorphism S S S o β β – malaria resistance, but still have sickle cell anemia and die young, often before reproducing (loss of β allele from population) o β β – die from malaria at a high frequency (loss of β allele from population) o β β – malaria resistance and no sickle cell anemia (gain of β and β allele in population)  heterozygote advantage S  The frequency of the β allele rises to a point at which it is counterbalanced by loss of the allele due to premature death by SCD Allele Frequency  Gene pool – all possible alleles  Allele frequency – the frequency of a single allele of a gene within the whole population o Frequency of blue allele of f(blue) = 30/100 = 30% or 0.3 o Frequency of green allele of f(green) = 70/100 = 70% or 0.7  Calculate genotypic frequency first, then allele frequency o Eg/ A diploid individual carriesa pair of alleles for A gene; AA or Aa or aa  f (AA) = number of AA individuals/Total number  f (Aa) = number of Aa individuals/Total number  f (aa) = number of aa individuals/Total number  f (AA) + f (Aa) + f(aa) = 1 o Total number of individuals in a population = N o Total number of alleles in a population = 2N o The number of individuals who are specifically the AA genotypeAA n o F (allele) = number of copies of the alleles / number of copies of all alleles for that gene 1 BIO 2C03 2013 o p = f(A) = 2AA+ nAa 2n = nAA+ ½n Aan  p = f (A) = f (AA) + ½f(Aa) o q = f(a) = aa + Aa/ 2n = naa ½n /Aa  q = f(a) = f(aa) + ½f(Aa) o p + q = 1  Mendel: patterns of inheritance for a cross between two individuals are based upon frequencies of alleles in individuals  Eg/ In an individual with the genotype Bb o p(B) = 0.5 o p(b) = 0.5  Frequencies of two alleles in an entire population are unlikely to be the same  Eg/ f(B) = 0.8 f(b) = 0.2 o B = dominant allele for dark fur color in cats  BB and Bb have dark fur o b = recessive allele for brown fur  bb have brown fur o In a population 80% of gametes carry a B allele 20% of gametes carry a b allele o So o 96% black fur  64% of offspring will be BB  32% of offspring will be Bb o 4% brown fur  4% of offspring will be bb  Brown fur color won’t disappear from the population o Re-Calculate the “new” gene pool of the next generation o f(B) = 0.64 + ½(0.32) = 0.80 or 80%  BB: all gametes are B (100%)  Bb: half gametes are B(50%) o f(b) = 0.04 + ½0.32 = 0.20 or 20%  bb: all gametes are b (100%)  Bb: half of gametes are b (50%) o Same allele frequencies maintained Hardy-Weinberg Equilibrium  Hardy-Weinberg Principle/Equilibrium – allele frequencies maintained o Gene Pool – total number of alleles of a gene in a population (here we have two alleles in the gene pool) o p = f(B) = frequency of one allele o q = f(b) = frequency of alternate allele o p + q = 1 o p = fraction of population homozygous, BB o q = fraction of population homozygous, bb 2 BIO 2C03 2013 o 2pq = fraction of population heterozygous, Bb o (p+q) = p + 2pq + q2 o At equilibrium, the ratio is maintained:  BB : Bb : bb p : 2pq : q2 o Since p + q = 1  P + 2pq + q = 1 o We can use this to determine genotype frequencies or allele frequencies o Recessive alleles are not lost at equilibrium  Hardy-Weinberg – as long as certain conditions are met, gene frequencies and genotype ratios in a randomly-breeding population will remain constant from one generation to the next  Populations are able to maintain a reservoir of variability so that if future conditions require it, the gene pool can change  Equilibrium is maintained given the following assumptions o No mutation o No gene flow o No genetic drift o Random mating o No natural selection  Mutation – can create new alleles or alter allele frequencies o No mutations – the composition of the gene pool remains the same generation after generation, if the other HWE conditions are met o Mutations – change the composition of the gene pool; new alleles introduced and allelic frequencies change  Gene Flow – different and isolated populations can have distinct gene pools (alleles and allele frequencies); if individuals from different populations are introduced, they can introduce new alleles or alter gene frequencies o No Migration – isolation of a population of trees prevents changes in the gene pool due to immigration and emigration o Migration – immigration of alleles in pollen from a neighboring population of trees can cause a change in the composition of the gene pool  Genetic Drift – particularly in small populations, chance alone may lead to the loss of individuals, changing allele frequencies o Large Breeding Population – an earthquake that kills 3 out of 10 million people, has little effect on the composition of the gene pool o Small Breeding Population – an earthquake that kills 3 out of 20 people has a significant effect on the composition of the gene pool  Non-random Mating – if individuals exhibit a preference in mate selection, gene frequencies may change (sexual selection) o Random Mating – coral polypsdisperse their sperm into the ocean currents; contact with an egg in another coral is completely up to chance o Assortive Mating – blister beetles are most likely to mate with partners of the same size 3 BIO 2C03 2013
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