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Genetics and Populations

Genetics and Populations. Chapter 14. 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

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Genetics and Populations

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  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. Case A: A Marker in the Blood • Certain small villages in Venezuela have a higher frequency of Huntington disease (HD) • Nancy Wexler collected information and created pedigrees (~19,000 people) • Collected blood samples to test DNA • Identified DNA marker in people with HD

  4. 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

  5. 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

  6. 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

  7. Frequency of Sickle Cell

  8. 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

  9. The Anopheles Mosquito

  10. 14.3 Specific Genetic Traits • Carrier frequency: Some populations have higher frequency of carriers of recessive traits • Differencesamong different populations • Two carriers from a high-risk population have a child, increased chance of genetic disorder

  11. Frequencies of Carriers of Tay-Sachswithin Populations

  12. 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

  13. 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

  14. Hardy-Weinberg Law to Study Genes in Populations • p represent A and q represent a • Only possible genotypes in a population • AAp2 (p X p = p2) • Aa or aA 2pq (pq X qp) • aaq2 (q X q = q2) • Sum of three genotypes must equal 100% • Therefore, p2 + 2pq + q2 = 1

  15. 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 • Used to calculate frequency of: • Disease-causing alleles in these populations • Heterozygotes in population

  16. Calculation of Allele Frequencies and Heterozygote Frequencies • Frequency of CF allele (c) in population: • Calculate number of people who have CF (genotype cc): • Frequency of CF = 1/2500 = 0.0004 • Per Hardy-Weinberg law, cc = q2 (q = frequency of CF) • Therefore genotype cc = q2 = 0.0004. • q = square root of 0.0004 = 0.02 • Therefore 2% of alleles in population are mutant CF allele (c)

  17. Animation: Allele and genotype frequencies

  18. 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

  19. Spotlight on Society: Heritage of African Tribe Revealed • Lemba people, in southern Africa • Oral tradition of ancestor Jews who left Middle East thousands of years ago • Culture very similar to Orthodox Judaism • ID pattern of molecular markers on Y chromosome, Cohen Model Haplotype

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