Chapter 13

1. Chapter 13 When Allele Frequencies Stay Constant

2. Population • A population is any group of members of the same species in a given geographical area. • Gene pool refers to the collection of all alleles in the members of the population. • Population genetics refers to the study of the genetics of a population and how the alleles vary with time.

3. Allele Frequencies • # of particular allele /total # of alleles • count both chromosomes of each individual • Allele frequencies affect the genotype frequencies or the frequency of each type of homozygote and heterozygote in the population.

4. Phenotype frequencies • Frequency of a trait is determined from observation • and varies in different populations.

5. Allele frequencies can change creating microevolution Conditions in which allele frequencies can change: • Individuals of one genotype reproduce more often with each other • Individuals migrate between populations • Population size is small or a group becomes reproductively isolated within a larger population • Mutation introduces new alleles or new copies of alleles • Individuals with a particular genotype are more likely to have viable, fertile offspring Nonrandom mating Migration Genetic drift Mutation Selection

6. Hardy-Weinberg Equilibrium • A condition in which allele frequencies remain • constant is called Hardy-Weinberg equilibrium Godfrey Harold Hardy, a mathematician, Wilhem Weinberg, a physician Used algebra to explain how allele frequencies can predict genotype and phenotype frequencies. “I am reluctant to intrude in a discussion concerning matters of which I have no expert knowledge, and I should have expected the very simple point which I wish to make to have been familiar to biologists.” -Godfrey Harold Hardy, Science, 1908

7. All of the allele frequencies together equals 1 or the whole collection of alleles All of the genotype frequencies together equals 1 Hardy-Weinberg Equilibrium • A condition in which allele frequencies remain • constant is called Hardy-Weinberg equilibrium p + q = 1 p allele frequency of one allele q allele frequency of a second allele p2 + 2pq + q2 = 1 p2 andq2 genotype frequencies for each homozygote 2pq genotype frequency for heterozygotes

9. Gamete frequencies .49 .21 .21 .09 Frequency D gamete= .7 frequency d gamete = .3 Hardy-Weinberg Equilibrium Generation 1 p allele frequency of D normal finger length = .7 q allele frequency of d short middle finger = .3 Genotype frequencies DD p2 = (.7)2 = .49 Dd 2pq = 2 (.7)(.3) = .42 dd q2 = (.3)2= .09

10. Male gametes d q=.3 D p=.7 Female gametes D p=.7 d q=.3 Hardy-Weinberg Equilibrium Generation 1 p allele frequency of D normal finger length = .7 q allele frequency of d short middle finger = .3 Frequency D gamete= .7 frequency d gamete = .3 DD p2=.49 Dd pq=.21 Dd pq=.21 dd q2=.09

11. Generation 2 Genotype frequencies DD = .49 Dd = .42 dd= .09 Hardy-Weinberg Equilibrium Generation 1 p allele frequency of D normal finger length = .7 q allele frequency of d short middle finger = .3 Allele frequencies p allele frequency of D normal finger length = .7 q allele frequency of d short middle finger = .3

12. q2 = .0005 = q2=.022 = 1 - q =.977 1 in 23 Application of Hardy-Weinberg Equilibrium:calculating risk Risk of being a carrier of cystic fibrosis for an Caucasian American depends upon Frequency of disease in population = 1 / 2000 Frequency of CF disease allele =q Frequency of wildtype CF allele =p p+q= 1, so Frequency of being heterozygote = 2pq = 2 (.977)(.022) = .043

13. Carrier Frequency for Cystic Fibrosis

14. All of the women in the population All of the men in the population Application of Hardy-Weinberg Equilibrium:calculating risk with X-linked traits p2 + 2pq + q2 = 1 Females: Males: p + q = 1 Hemophilia is X-linked and occurs in 1 in 10,000 males p= 1/10,000 = .0001 q= .9999 Carrier females = 2pq = 2 (.0001) (.9999) = .0002 1 in 5000 are carriers Affected females = p2 = (.0001) 2 = .00000001 1 in 100 million women will have hemophilia

15. Application of Hardy-Weinberg Equilibrium:DNA identification Variationin DNA sequences within a population can be used to determine the probability that a sample came from an individual. Tissue commonly tested: Skin cells Buccal cells in saliva White blood cells in blood Follicle cells on hair Semen

16. Application of Hardy-Weinberg Equilibrium:DNA identification Identity: How likely is it that the sample came from a particular person (accident victim, crime suspect) Relationship: How likely is it that two individuals are related (paternity cases, missing persons)

17. Application of Hardy-Weinberg Equilibrium:DNA identification SNPs or Single nucleotide polymorphisms Single base differences between chromosomes Repeated sequences Variation in the number of repeats present Variation in DNA sequences outside of genes are subject to Hardy-Weinberg equilibrium. Noncoding variation is useful as it is not subject to as many impacts that lead to deviations in H-W equilibrium namely selection and assortative mating.

18. Application of Hardy-Weinberg Equilibrium:DNA identification

19. Application of Hardy-Weinberg Equilibrium:DNA identification Restriction Fragment Length Polymorphism analysis or RFLP analysis is one method for detecting variation. A restriction enzyme cuts the DNA at a particular recognition sequence. The pattern of DNA fragments indicates the presence of DNA polymorphisms.

20. Application of Hardy-Weinberg Equilibrium:DNA identification Restriction enzymes cut DNA => Electrophoresis separates DNA fragments by size

21. Application of Hardy-Weinberg Equilibrium:DNA identification

22. Application of Hardy-Weinberg Equilibrium:DNA identification

23. Application of Hardy-Weinberg Equilibrium:DNA identification • The probability of obtaining the DNA pattern observed from the population is calculated using the frequency of alleles in that population. • Genotype frequencies of unlinked loci are multiplied. • Allele frequencies vary in different populations. Use of correct population information is important.