1. Population Genetics and Evolution
Mr. Nichols PHHS 1

2. Cartoons of the Day!

3. Cartoons of the Day!

4. Cartoons of the Day!

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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

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

14. 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

15. 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)

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

17. 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

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

19. 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

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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

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

29. 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

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

31. 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

32. 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)

33. Tristan da Cuhna
An island population founded by a single family

34. 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

35. 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

36. 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

37. 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

38. 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

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

40. 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

41. 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

42. 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

43. 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

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

45. 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

46. 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

47. 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