1. Genetics and Populations Chapter 14

2. Central Points  Genetic conditions can be very common in a specific community  Huntington disease affects large numbers in two villages in Venezuela  Traits can vary from one population to another  Calculations can determine frequency of an allele  Population genetics used in DNA forensics

3. 14.1 Why Study Populations?  Small isolated populations often have a high frequency of one or more genetic disorders  Pedigrees, blood and tissue samples used to identify, map, and isolate genes responsible  Pedigrees trace HD mutation to one woman  Founder effect

4. Repeated DNA Triplet Causes Huntington Disease (HD)  Normally, 10–35 copies of CAG repeat, < 27 CAG repeats do not get HD  27–35 copies do not get HD, but children at risk  36–40 copies may or may not get HD  > 40 repeats almost always get HD  Increase of number of repeats each generation

5. 14.2 Other Genetic Disorders  Geographic distribution of sickle cell anemia and malaria  Link between sickle cell anemia and malaria  Malaria affects > 500 million people worldwide and kills > 3 million people/year  Caused by parasite, infects red blood cells

6. Frequency of Sickle Cell

7. Malaria and Sickle Cell Anemia  Spread by mosquitoes  Carriers of sickle cell anemia (heterozygotes) and affected individuals (homozygotes), resistant to infection by malaria parasite  Membrane of red blood cells altered, very difficult for parasite to enter cells

8. The Anopheles Mosquito

9. 14.3 Specific Genetic Traits  Carrier frequency: Some populations have higher frequency of carriers of recessive traits  Differences among different populations  Two carriers from a high-risk population have a child, increased chance of genetic disorder

10. Frequencies of Carriers of Tay-Sachs within Populations

11. 14.4 Environmental Conditions Affect Frequency of Genetic Traits in Populations?  Cystic fibrosis (CF) common in some populations but nearly absent in others  CF affects glands that produce mucus, digestive enzymes, and sweat, causing far-reaching effects  Most individuals with CF develop obstructive lung disease and infections, leading to premature death

12. Cystic Fibrosis Centers

13. CF and Typhoid Fever  Previously, affected individuals usually died before having children  Some evidence that heterozygotes more resistant to typhoid fever  Caused by a bacterium that infects cells of intestinal lining  In mice, carriers of CF injected with typhoid fever, intestinal cells infected by fewer bacteria

14. 14.5 Frequency of Alleles in a Population  Genetic disorder caused by recessive allele  Cannot directly count those who carry allele in population (cannot ID heterozygote, Cc)  Hardy and Weinberg developed formula, measures numbers of alleles and genotypes in a population

15. Hardy-Weinberg Law to Study Genes in Populations  p represent A and q represent a  Only possible genotypes in a population AA p 2 (p X p = p 2 ) Aa or aA2pq (pq X qp) aa q 2 (q X q = q 2 )  Sum of three genotypes must equal 100%  Therefore, p 2 + 2pq + q 2 = 1

16. How Can We Use the Hardy-Weinberg Law?  Frequency of alleles and genotypes population provides information  Risk factors for having child affected with genetic disorder  To determine if populations are evolving  Used to calculate frequency of: Disease-causing alleles in these populations Heterozygotes in population

17. Calculation of Allele Frequencies and Heterozygote Frequencies  Frequency of CF allele (c) in population: 1.Calculate number of people who have CF (genotype cc): 2.Frequency of CF = 1/2500 = 0.0004 3.Per Hardy-Weinberg law, cc = q 2 (q = frequency of CF) 4.Therefore genotype cc = q 2 = 0.0004. 5.q = square root of 0.0004 = 0.02  Therefore 2% of alleles in population are mutant CF allele (c)

18. Animation: Allele and genotype frequencies

19. Other Uses of Hardy-Weinberg Equation  No malaria in U.S., but individuals with West African ancestry carry sickle cell gene  Frequency of children with sickle cell anemia: Use Hardy-Weinberg to calculate frequency of carriers (Ss) ~8% or 1/12 African Americans with West African ancestry  Some areas of West Africa: 20–40% of population are carriers (Ss) of sickle cell gene

20. Animation: The Hardy-Weinberg equation

21. 14.6 Legal and Ethical Issues (1)  Construction and use of DNA databases  Who should be forced to provide sample?  Who has the authority to order sample?  Should DNA profiles of those found innocent remain in database?  What crimes should be included in database?

22. 14.6 Legal and Ethical Issues (2)  What about private information unrelated to crimes  What about privacy rights?  Does DNA data show the criminal justice system is racially or ethnically biased?