Population Genetics and Evolution Mr. Nichols PHHS 1

Cartoons of the Day!

Cartoons of the Day!

Cartoons of the Day!

The Population as a Genetic Reservoir Humans are not distributed randomly across the world, but are clustered into discrete populations

Populations Populations Local groups of organisms belonging to a single species, sharing a common gene pool Populations can be described by age structure, geography, birth and death rates, and allele frequencies

Population Diversity Populations are more diverse than individuals Only a group can carry all the alleles for traits such as blood types A, B, AB, and O All the alleles in a population are the gene pool Gene pool The set of genetic information carried by the members of a sexually reproducing population

Allele Frequency Allele frequency The frequency with which alleles of a particular gene are present in a population The frequency of alleles in a population may change from generation to generation Changes in allele frequency can cause change in phenotype frequency; long-term change in allele frequency is evolutionary change

How Can We Measure Allele Frequencies in Populations? Population genetics studies allele frequencies in populations, not offspring of single matings In some cases allele frequency in a population can be measured directly In other cases, the Hardy-Weinberg Law is used to estimate allele frequencies within populations

Codominant Allele Frequencies Can Be Measured Directly Codominant allele frequencies can be measured directly by counting phenotypes Phenotypes are equivalent to genotypes Example: The MN blood group LM and LN alleles are codominant and produce three phenotypes, M, N, and MN

Recessive Allele Frequencies Cannot Be Measured Directly With recessive alleles, there is no direct relationship between phenotype and genotype Heterozygotes and dominant homozygotes have the same phenotype Hardy and Weinberg independently developed a mathematical formula to determine frequency of alleles when one or more alleles are recessive Under certain specified conditions

The Hardy-Weinberg Law Measures Allele and Genotype Frequencies Allele and genotype frequencies remain constant from generation to generation when the population meets certain assumptions There is a difference between how a trait is inherited and the frequency of recessive and dominant alleles in a population

Brachydactyly: A Dominant Trait What is the relationship between allele frequency and phenotype frequency?

Assumptions of the Hardy-Weinberg Law The population is large enough that there are no errors in measuring allele frequencies All genotypes are equally able to reproduce Mating in the population is random Other factors that change allele frequency (mutation and migration) can be ignored

Mathematics of the Hardy-Weinberg Law For a population, p + q = 1 p = frequency of the dominant allele A q = frequency of the recessive allele a The chance of a fertilized egg carrying the same alleles is p2 (AA) or q2 (aa) The chance of a fertilized egg carrying different alleles is pq (Aa)

Determining the Frequency of Alleles in a New Generation Depends on the frequency of dominant and recessive alleles in the parental generation

The Hardy-Weinberg Equation p2 + 2pq + q2 = 1 1 = 100% of genotypes in the new generation p2 and q2 are the frequencies of homozygous dominant and recessive genotypes 2pq is the frequency of the heterozygous genotype in the population

Calculating Frequency of Alleles in a New Generation Given alleles in the parental generation are a (q = 0.4) and A (p = 0.6)

Populations Can Be in Genetic Equilibrium When the allele frequency for a particular gene remains constant from generation to generation Equilibrium in a population explains why dominant alleles do not replace recessive alleles In equilibrium populations, Hardy-Weinberg law can be used to measure allele and genotype frequencies from generation to generation

Keep In Mind The frequency of recessive alleles in a population cannot be measured directly

Using the Hardy-Weinberg Law in Human Genetics The Hardy-Weinberg Law can be used to Estimate frequencies of autosomal dominant and recessive alleles in a population Detect when allele frequencies are shifting in a population (evolutionary change) Measure the frequency of heterozygous carriers of deleterious recessive alleles in a population

Calculating the Frequency of Autosomal Dominant and Recessive Alleles Count the frequency of individuals in the population with the recessive phenotype, which is also the homozygous recessive genotype (aa) The frequency of genotype aa = q2 The frequency of the a allele is √q2 = q The frequency of the dominant allele (A) is calculated p = 1 - q

Calculating the Frequency of Alleles for X-Linked Traits For X-linked traits, females (XX) carry 2/3 of the alleles and males (XY) carry 1/3 of the alleles The number of males with the mutant phenotype equals the allele frequency for the recessive trait Frequency of an X-linked trait in males is q Frequency of the trait in females is q2

Calculating the Frequency of Multiple Alleles In ABO blood types, six different genotypes are possible (AA, AO, BB, BO, AB, OO) Allele frequencies: p (A) + q (B) + r (O) = 1 Genotype frequencies: (p + q + r)2 = 1 Expanded Hardy-Weinberg equation: p2 (AA) + 2pq (AB) + 2pr (AO) + q2 (BB) + 2qr (BO) + r2 (OO) = 1

Frequencies of Heterozygotes For a genetic disorder inherited as a recessive trait, most disease-causing alleles are carried by heterozygotes The frequency of heterozygous carriers of deleterious recessive alleles in a population is used to calculate risk of having an affected child

Estimating the Frequency of Heterozygotes in a Population Count the number of homozygous individuals in the population (q2) and calculate the frequency of the recessive allele q Calculate the frequency of the dominant allele p (p = 1- q) Calculate the frequency for the heterozygote genotype 2pq

Relationship between Allelic Frequency and Genotype Frequency What are the chances of two heterozygotes mating and having a child with a recessive trait? If 1 in 10,000 members of the population have the disorder, then 1 in 50 is a heterozygote Chance of two mating is 1/50 x 1/50 = 1/2,500 Chance of a given child being affected is ¼ Chance of mating and having an affected child is 1/2,500 x ¼ = 1/10,000

Keep In Mind Estimating the frequency of heterozygotes in a population is an important part of genetic counseling

Measuring Genetic Diversity in Human Populations Human populations carry a large amount of genetic diversity Mutation generates new alleles, but has little impact on allele frequency If the mutation rate for a gene is known, the change in allele frequency resulting from new mutations in each generation can be calculated

Replacement of a Recessive Allele by Mutation Alone Mutation alone has a minimal impact on the genetic variability present in a population

Genetic Drift Can Change Allele Frequencies Forces such as genetic drift act on the genetic variation in the gene pool to change the frequency of alleles in the population Genetic drift Random fluctuations of allele frequencies from generation to generation that take place in small, isolated populations such as island populations or socioreligious groups

Founder Effects Occasionally, populations start with a small number of individuals (founders) Founder effects Allele frequencies established by chance in a population that is started by a small number of individuals (perhaps only a fertilized female)

Tristan da Cuhna An island population founded by a single family

Tristan da Cuhna Residents of the island of Tristan da Cuhna are an example of isolation and inbreeding Show increased homozygosity for recessive traits such as clinodactyly Clinodactyly An autosomal dominant trait that produces a bent finger

Selection Wallace and Darwin identified selection as the primary force that leads to evolutionary divergence and the formation of new species Selection increases the reproductive success of fitter genotypes

Natural Selection Acts on Variation in Populations Natural selection acts on genetic diversity in populations and is the major force in driving evolution Natural selection Differential reproduction shown by some members of a population that is the result of differences in fitness

Fitness Better-adapted individuals have an increased chance of leaving more offspring Fitness A measure of the relative survival and reproductive success of a specific individual or genotype

The Relationship between Sickle-Cell Anemia and Malaria The allele for sickle-cell anemia is present in very high frequencies in certain populations Many recessive homozygotes die in childhood The sickle-cell allele confers resistance to the parasite Plasmodium, which causes malaria Selection favors survival and differential reproduction of heterozygotes

Keep In Mind Mutation generates all new alleles, but drift, migration, and selection determine the frequency of alleles in a population

Natural Selection Affects the Frequency of Genetic Disorders Rare lethal or deleterious recessive alleles survive because the vast majority of them are carried in the heterozygous condition Other factors can cause differential distribution of alleles in the human population Migration, founder effects, mutations, selection

Lethal Alleles Almost all individuals with Duchenne muscular dystrophy (DMD) die before reproducing The mutation rate for DMD is high, introducing more DMD alleles The frequency of the DMD allele in a population is balanced between alleles introduced by mutation and those removed by deaths

Heterozygote Advantage The high frequency of genetic disorders in some populations is the result of selection that often confers increased fitness on heterozygotes A single sickle-cell allele confers resistance to malaria A single Tay-Sachs allele confers resistance to tuberculosis

Genetics in Society: Lactose Intolerance and Culture The enzyme lactase converts lactose (milk sugar) into glucose and galactose Lactase production slows or stops after childhood Some populations have a gene for adult lactose metabolism (LA) The cultural practice of keeping dairy herds was a selective factor that provided an advantage for the LA genotype

Keep In Mind Survival and differential reproduction are the basis of natural selection

Genetic Variation in Human Populations The biological concept of race changed from an emphasis on phenotypic differences to an emphasis on genotypic differences Mutation introduces genetic variation Natural selection and drift are the primary mechanisms that spread alleles through local population groups

How Can We Measure Gene Flow Between Populations? Gene flow between populations is used to reconstruct the origin and history of populations Example: Gene flow into the American black population from Europeans West African populations have blood group FY*O Europeans have blood groups FY*A and FY*B In northern US cities, about 20% of genes in the black population are derived from Europeans

Are There Human Races? Studies of variations in proteins, microsatellites, and nuclear genes show more genetic variation within populations than between populations Conclusion: There is no clear genetic basis for dividing our species into races