Chapter 14

1. Chapter 14 Changing Allele Frequencies

2. 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

3. Nonrandom mating • Random mating occurs when individuals of one genotype mate randomly with individuals of all other genotypes. • Nonrandom mating indicates individuals of one genotype reproduce more often with each other • Ethnic or religious preferences • Isolate communities • Worldwide, 1/3 of all marriages are between people born within 10 miles of each other • Cultures in which consanguinity is more prominent • Consanguinity is marriage between relatives • e.g. second or third cousins

4. Original population Population after migration 2 3 25% (5/20) 13.3% (2/15) 25% (5/20) 33.3% (5/15) 25% (5/20) 20% (3/15) 25% (5/20) 33.3% (5/15) Migration • When people move, their genes go with them. • The genetic effects of migration are reflected in current populations. • Allele frequencies change when alleles are disproportionately distributed in the migrants.

5. Migration • Changes in allele frequency can be mapped across geographical or linguistic regions. • Allele frequency differences between current populations can be correlated to certain historical events.

6. Mapping a trait geographically can suggest patterns of migration • Frequencies of galactokinase deficiency decrease westward from home of the Viax Roma in Bulgaria. • Gradients in allele frequencies between successive neighboring populations are called clines.

7. Allele distributions can reflect historical events • Creutzfeldt-Jakob disease (CJD) is caused by a mutation in the prion protein • 70% of families with CJD share the same allele • Families from Libya, Tunisia, Italy,Chile and Spain share a common haplotype. • These populations were expelled from Spain in the Middle Ages.

8. Original population Population after genetic drift 25% (5/20) 30% (6/20) 25% (5/20) 30% (6/20) 25% (5/20) 20% (4/20) 25% (5/20) 20% (4/20) Genetic Drift • Changes in allele frequency occur when gametes do not reflect the allele frequencies in parents by random chance. • This random sampling error is called genetic drift. • Genetic drift is more pronounced in small populations.

9. Genetic Drift • Events that create small populations enhance the effect of genetic drift. • Founding a new population • Bottlenecks (natural disaster, famine) • Geographic separation (islands) • Linguistic differences

10. Genetic Drift • Founder effects refer to the genetic impact of starting a new population from a relatively small group of people.

11. Founder effects can alter allele frequencies • Dunkers are descendants of German immigrants who emigrated to Germantown, Pennsylvania in 1719-29

12. Genetic drift and nonrandom mating • Small population size increases the probability of homozygosity for recessive alleles present in the population. • Amish and Mennonite populations marry predominantly within their religious groups, which coincidentally maintains their original small genetic pool. • Increased incidence of otherwise rare traits is observed. Amish women and child with Ellis-van Creveld syndrome

13. Genetic drift • A population bottleneck occurs when a large population is drastically reduced in size. • Rebounds in population size occur with descendants of a limited number of survivors.

14. Original population Population after mutation 25% (5/20) 30% (6/20) 25% (5/20) 15% (3/20) 25% (5/20) 30% (6/20) 25% (5/20) 25% (5/20) Mutation • Allele frequencies change in response to mutation. • Mutation can introduce new alleles. • Mutation can convert one allele to another. • Mutation has a minor impact unless coupled with another effect (small population size, selection).

15. Mutation • Allele frequencies change in response to mutation. • Mutation can introduce new alleles. • Mutation can convert one allele to another. • Mutation has a minor impact unless coupled with another effect (small population size, selection). Selection acts to eliminate deleterious alleles. • Dominant deleterious alleles disappear quickly. • Recessive deleterious alleles are eliminated when homozygotes appear and fail to reproduce. • The collection of recessive deleterious alleles present in a population is called the genetic load.

16. Original population Population after selection 33.3% (5/15) 25% (5/20) do not reproduce 33.3% (5/15) 25% (5/20) 0% (0/15) 25% (5/20) 33.3% (5/15) 25% (5/20) Natural Selection • The differential survival and reproduction of individuals with a particular phenotype is called natural selection. • Natural selection may result in an increase (positive seletion) or decrease (negative selection) in the frequency of an allele

17. Natural Selection • Tuberculosis (TB) infections have historically swept across susceptible populations killing many. • TB epidemic among Plains Indians • of Qu’Appelle Valley Reservation • annual deaths • 1880s 10 % • 1921 7 % • 1950 0.2% • Recent resurgence of TB worldwide reflects increasing bacterial resistance to antibiotics (1 in 7 cases). • Natural selection of the bacteria has occurred.

18. Natural Selection and HIV • Initially the immune system identifies and eliminates many cells infected with HIV. • Mutations occur in the virus. • Viral mutations allowing increased replication or evasion of the immune system are favored. • Gradually the immune system of the infected person can no longer fight off the HIV infection. • HIV infection progresses to AIDS when lack of an intact immune system leads to opportunistic infections. • HIV undergoes selection within a human host.

19. Natural Selection and HIV Treatment with multiple drugs limits viral diversification early in infection and prolongs progression of the disease to full blown AIDS.

20. Balanced Polymorphism • When two or more forces act in different • directions on alleles of a gene it is called • balanced polymorphism. The beta hemoglobin gene exhibits balanced polymorphism HB S allele causes the recessive sickle cell anemia trait. The HB S allele is under negative selection. HB S allele helps protect heterozygotes from malaria. The HB S allele is under positive selection. This is also called heterozygote advantage.

21. Sickle cell anemia and malaria produce opposing selection on hemoglobin alleles

22. Balanced polymorphisms are often associated with resistance to infection

23. Beneficial traits can become deleterious in different conditions • MDR1 gene encodes a membrane protein that pumps foreign molecules out of a cell. • helps remove toxins, an advantage historically • removes drugs used to treat AIDS, a disadvantage now

24. Forces that alter allele frequencies

25. Gene Histories: PKU A unique PKU allele originated in San’a and spread among Yemenite Jews.

26. Gene Histories: cystic fibrosis The most common cystic fibrosis allele is DF508 indicating it may be an old mutation predating European expansion.