Understanding Allele Frequencies and Hardy-Weinberg Equilibrium in Population Genetics

1. Chapter 12 When Allele Frequencies Stay the Same..

2. Population Genetics • Branch of genetics that considers not individuals or families, but ALL of the alleles in a population. Gene Pool

3. Gene Flow

4. 12.1 Importance of Knowing Allele Frequencies • Frequencies can be calculated for alleles, genotypes, or phenotypes.

5. Phenotypic Frequencies • Determined empirically (observing how common a condition is in a population). • Data supplied to genetic counselors

6. Shifting allele frequencies in populations provide the small steps of genetic change = microevolution • Frequencies change when any of the following occur: • Nonrandom Mating • Migration of individuals • Reproductively isolated subgroups form within a larger population • Mutation occurs • Natural Selection

7. Hardy-Weinberg Equilibrium • Hardy says, “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.” • He was hearing biologist assume that dominant traits will tend to increase in populations, while recessive traits will become rarer

8. The famous equation… • p² + 2pq + q² = 1.0 • P² = homozygous dominant individuals • 2pq = heterozygous individuals • q² = homozygous recessive individuals • The idea is that if the proportion of genotypes remains the same from gen to gen, then that gene is not evolving!

9. It’s rare but it is seen… • Rare for protein encoding genes that affect phentoype. • Occurs in repeated sequences that do NOT affect phenotypes – therefore not acted upon by Natural Selection

10. Solving Hardy Weinberg Equilibrium • Within a population, the easiest to value to determine is q². • q² q p p² 2pq • Within a population of butterflies, the color brown (B) is dominant over the color white (b). And, 40% of all butterflies are white. Given this simple information, which is something that is very likely to be on an exam, calculate the five variables:

11. Practical Application of Hardy-Weinberg Equilibrium • Truly determining risk for a couple. • What are the chances my child will have cystic fibrosis? Known: Cystic fibrosis affects 1 in 2,000 Caucasians! 2pq! 1/2000 = 0.0005 q² q p

12. .043 1 ??

13. 1 1 1 4 23 23 Why?!?

14. 12.4 – DNA Fingerprinting – A practical test of Hardy Weinberg Assumptions • Polymorphism – a site on a chromosome that varies within at least 1% of the population • Can be as small as one nucleotide (Snp) • The ability to distinguish people based upon polymorphisms = basis of DNA Fingerprints • Restriction Enzymes • Spots polymorphisms

15. DNA Fingerprinting and Hardy Weinberg Variations of a gene are caused by altered proteins Different Proteins = Different Sizes Different sizes travel different lengths

17. If the frequency of +/+ (p²) = 1/3,000 then a DNA fingerprint will run multiple genes of known statistics!

18. What are the chances another person did it??

19. Chapter 13 Changing Allele Frequencies

20. Nonrandom Mating • Surveys show we marry people similar to ourselves 80% of the time. • Worldwide, 1/3 of all marriages occur between people who were born fewer than 10 miles apart • Prize bull can fertilize thousands of cows • Chinese Immigrant to South Africa – 7 wives (he had a dominant disorder – teeth fall out) • 356 living descendents • 70 have the dental disorder

21. Nonrandom Mating • 1 in 200 Hopi Indians are albino. How can the culture of Indians cause the trait to be so prevalent? • Mutation can cause individuals to be resistant to HIV. • How would this mutation cause a change in Hardy Weinberg equilibrium over time? • Would you expect blood type frequencies to be in Hardy Weinberg equilibrium? Why or why not?

22. Nonrandom Mating • How can “Cystic Fibrosis Summer Camps” be a detriment to Hardy Weinberg equilibrium? • In a very religious Jewish sect in New York City, marriages are NOT permitted between carriers of the same inherited disease • Arranged marriages AFTER genetic testing • How does a consanguineous population affect the equilibrium? • Population of Sardinia (island near Italy) – 86% of the individuals have the same X chromosome.

23. Migration • How can allele frequencies tell a story of the past?

24. Geographical Clues • Allele frequencies that vary between neighboring populations = Cline • Cline gradients can be gradual • Steep gradients often are a result of geographical barriers • Evidence: The fertile lands along the Nile River • Mummies

25. Linguistic Clues • Italy • Is the changing frequencies due to geography or language? • Blood groups study across the region of Italy • Language groups were more closely related than Geography based.

26. Small Population • Genetic Drift – the random chance of alleles being passed on to the next generation

27. Founder Effect • When a small population leaves another and starts a new colony. The new colony will develop a different allele frequency than the original.

28. Pennsylvania is a breeding ground… • Germantown and The Dunkers • Lancaster County Amish • 11 different disorders are more prevalent amongst the Amish compared to the US population

29. Haplotypes • Haplotype – a group of VERY tightly linked genes on a chromosome • Genetic drift leads to common diseases amongst populations but also other genes that are close to that locus.

30. Population Bottleneck

31. Mutation • Many mutations in homozygous forms are lethal, but since they can be hidden in heterozygous the allele can be passed on. • Genetic Load – the collection of deleterious alleles in a population.

32. Natural Selection and Heterozygote Advantage