武汉大学：《遗传学》课程教学课件（英文讲稿）Chapter 20 Variation and Selection in Populations

Chapter 20 Variation and Selection in Populations 21-1

Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 21-1 Variation and Selection in Populations Variation and Selection in Populations Chapter 20

Sectionsto study20.1 The Hardy-Weinberg law:Predictinggenetic variation in“ideal"populations20.2 What causes allele frequency to change in real populations?20.3 Ancestry and the evolution of modern humans21-2

Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 21-2 Sections to study Sections to study 20.1 The Hardy 20.1 The Hardy-Weinberg law: Predicting genetic variation in Weinberg law: Predicting genetic variation in “ideal” populations populations 20.2 What causes allele frequenc 20.2 What causes allele frequency to change in real populations? y to change in real populations? 20.3 Ancestry and the evolution of modern humans 20.3 Ancestry and the evolution of modern humans

Prior to the 2Oth century, many people thought that “recessive"phenotypes such as naturally blonde or red hair would becomeextinct over timein human population.The blended inheritance theory claims thatthe information for blondeand red hair would be lost gradually when people with lighter hair colormated with peoplewith darkerhair.In1908,GeoffreyH.HardyandWilhelm Weinberg proposedthatif certain assumptions weremet, these phenotypes wouldremain constant overtime and between generations.21-3

Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 21-3  Prior to the 20th century, many people thought that Prior to the 20th century, many people thought that “recessive recessive” phenotypes such as phenotypes such as naturally blonde naturally blonde or red hair red hair would become would become extinct over time in human population. extinct over time in human population.  The blended inheritance theory claims that the information for b The blended inheritance theory claims that the information for blonde and red hair would be lost gradually when people with lighter ha and red hair would be lost gradually when people with lighter hair color ir color mated with people with darker hair. mated with people with darker hair.  In 1908, Geoffrey H. Hardy and Wilhelm Weinberg proposed In 1908, Geoffrey H. Hardy and Wilhelm Weinberg proposed that if certain assumptions were met, these phenotypes would that if certain assumptions were met, these phenotypes would remain constant remain constant over time over time and between generations between generations

Population genetics: The scientific discipline that studies whathappens in wholepopulations at thegenetic level.Population geneticists rely on mathematical models in predicting apopulation's potential for stasis orchange.Simple models help clarifying the guestions about freguency of geneticdiseases orrate of spread ofpathogens,as well as making predictionsaboutfuturepopulations.21-4

Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 21-4  Population genetics Population genetics: The scientific discipline that studies what : The scientific discipline that studies what happens in whole populations at the genetic level. happens in whole populations at the genetic level.  Population geneticists rely on mathematical models in predicting Population geneticists rely on mathematical models in predicting a population population’s potential for stasis or change. s potential for stasis or change.  Simple models help clarifying the questions about frequency of g Simple models help clarifying the questions about frequency of genetic diseases or rate of spread of pathogens, as well as making predi diseases or rate of spread of pathogens, as well as making predictions about future populations. about future populations

20.1 The Hardy-Weinberg law: Predicting geneticvariationin“ideal"populationsPopulation:A group ofinterbreeding individualsof the same species thatinhabit the same space atthe sametime.AlsocalledMendelian populationGirrafe herd on the African savanna21-5

Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 21-5  Population Population: A group of : A group of interbreeding individuals interbreeding individuals of the same species that of the same species that inhabit the same space at inhabit the same space at the same time. Also called the same time. Also called Mendelian Mendelian population population. Girrafe herd on the African savanna 20.1 The Hardy-Weinberg law: Predicting genetic variation in “ideal” populations

Gene pool:The sum total of all alleles carried in all members ofapopulation.Agenepool represents all of thealleles present onthe chromosomes ofall members of a population and the relativeprominence or rarenessofeachallele.Agenepool canchangeovertime.New alleles arise by mutation or are introduced by immigration.Rare pre-existing alleles disappear when all individuals carrying themleavethepopulationordie.Microevolution: Alterations of a population's gene pool21-6

Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 21-6  Gene pool Gene pool: The sum total of all alleles carried in all members of : The sum total of all alleles carried in all members of a population. a population.  A gene pool represents all of th A gene pool represents all of the alleles present on the chromos e alleles present on the chromosomes of omes of all members of a population and the relative prominence or raren all members of a population and the relative prominence or rareness of each allele. of each allele.  A gene pool can change over time. A gene pool can change over time.  New alleles arise by mutation or are introduced by immigration. New alleles arise by mutation or are introduced by immigration.  Rare pre Rare pre-existing alleles disappear when all individuals carrying them existing alleles disappear when all individuals carrying them leave the population or die. leave the population or die. Microevolution Microevolution: Alterations of a population : Alterations of a population’s gene pool. s gene pool

Apopulation is defined by its genotype frequencies and allelefrequencies, which together make up a gene pool.Phenotype frequency:The proportion of individuals in apopulation thatexpress aparticularphenotypeGenotype frequency:The proportion of total individuals in apopulation that carrya particular genotype.Allele frequency:The proportion of gene copies in a wholepopulation that are of a given allele type.21-7

Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 21-7  Phenotype frequency Phenotype frequency: The proportion of individuals in a : The proportion of individuals in a population that express a particular phenotype. population that express a particular phenotype.  Genotype frequency Genotype frequency: The proportion of total individuals in a : The proportion of total individuals in a population that carry a particular genotype. population that carry a particular genotype.  Allele frequency Allele frequency: The proportion of gene copies in a whole : The proportion of gene copies in a whole population that are of a given allele type. population that are of a given allele type. A population is defined by its genotype frequencies and allele frequencies, which together make up a gene pool

Phenotype frequency: The proportion of individuals in apopulation that express a particular phenotype.Example:In apopulation of20humanindividuals,4peoplehaveblue-coloredeyes becausethey are homozygousfor the recessive Ballele at a particular "blue eyes" locus, where the alternative alleleis A.Molecular analyses showed that 12 individuals are of genotypeAA,4areofgenotypeAB.Phenotype frequency of blue eyes:4/20=0.2dark eyes: (20-4)/20=0.821-8

Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 21-8  Phenotype frequency Phenotype frequency: The proportion of individuals in a : The proportion of individuals in a population that express a particular phenotype. population that express a particular phenotype. Example: In a population of 20 human individuals, 4 people have blue-colored eyes because they are homozygous for the recessive B allele at a particular “blue eyes” locus, where the alternative allele is A. Molecular analyses showed that 12 individuals are of genotype AA, 4 are of genotype AB. Phenotype frequency of blue eyes: 4/20 = 0.2 dark eyes: (20-4)/20 = 0.8

Genotype frequency:The proportion oftotalindividuals in apopulation that carry a particular genotype.Example:In a population of 20 human individuals,4people haveblue-colored eyes because they are homozygous for the recessive Ballele at a particular "blue eyes" locus,where the alternative alleleis A.Molecular analyses showed that 12 individuals are of genotypeAA, 4areofgenotypeAB.Genotype frequency of AA: 12/20=0.6AB:4/20=0.2BB:4/20=0.221-9

Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 21-9  Genotype frequency Genotype frequency: The proportion of total individuals in a : The proportion of total individuals in a population that carry a particular genotype. population that carry a particular genotype. Example: In a population of 20 human individuals, 4 people have blue-colored eyes because they are homozygous for the recessive B allele at a particular “blue eyes” locus, where the alternative allele is A. Molecular analyses showed that 12 individuals are of genotype AA, 4 are of genotype AB. Genotype frequency of AA: 12/20 = 0.6 AB: 4/20 = 0.2 BB: 4/20 = 0.2

Allele frequency: The proportion of gene copies in a wholepopulation that are ofa given allele type.Example:In a population of 20 human individuals, 4 people haveblue-colored eyes because they are homozygous for the recessive Ballele at a particular "blue eyes" locus, where the alternative alleleis A.Molecular analyses showed that 12 individuals are of genotypeAA,4areofgenotypeAB.21-10

Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 21-10  Allele frequency Allele frequency: The proportion of gene copies in a whole : The proportion of gene copies in a whole population that are of a given allele type. population that are of a given allele type. Example: In a population of 20 human individuals, 4 people have blue-colored eyes because they are homozygous for the recessive B allele at a particular “blue eyes” locus, where the alternative allele is A. Molecular analyses showed that 12 individuals are of genotype AA, 4 are of genotype AB