2/19/2014 1 Course Biz • Midterm next week – Midterm will include everything to this point including today – Sample questions on Moodle this week – 50 multiple choice questions – Bring pencil and eraser 1 Lectures: • Intro • Evolution & Darwin’s Theories • Natural Selection • Genetics: Variation & Inheritance • Forces of Evolution • Speciation & Phylogeny • Human Variation • Population Genetics Labs: • The Human Skeleton • Natural Selection • Evolution & Genetics Textbook: • Introduction • Ch. 1 • Ch. 2 • Ch. 3 & App. B • Ch. 4 • Ch. 5 Human Variation [part 2] 22/19/2014 2 Today’s Objectives • Population genetics: genetic variation within and between groups of organisms • Polymorphisms – ABO blood type – Lactose intolerance – Sickle-cell • Other aspects of human variation 3 Polymorphism • Genetic trait governed by a locus with more than one allele • Two or more distinct phenotypes that exist within a population – Examples: ABO blood type and lactose tolerance • Balanced polymorphism – the maintenance of two or more alleles in a population due to the selective advantage of the heterozygote – Example: sickle-cell and lactose tolerance 42/19/2014 3 ABO blood type • Three alleles: A, B, O • Six genotypes: AA, AB, AO, BB, BO, OO • Four phenotypes: Type A, Type B, Type AB, Type O • Classic example of a polymorphic trait – at least two alleles present and the alleles are both present in the populations with a frequency greater than 1% 5 62/19/2014 4 7 82/19/2014 5 ABO blood type • Why are the polymorphisms maintained in different populations? • Why do we not see more alleles at fixation because of bottlenecks or genetic drift? 9 ABO blood type • A and B allele frequencies are always less than 50% • Research suggests infectious disease plays a key role in the allele distribution – Balance between A and B alleles is maintained in populations with a heavy load of bacterial disease, whereas O alleles predominates in populations that are more vulnerable to viral diseases 102/19/2014 6 Balanced Polymorphism Lactose Tolerance/Intolerance 11 Milk: good or bad? 122/19/2014 7 Percent lactose intolerant Population Group Percent U.S. (European ancestry) 2-19 U.S. (African ancestry) 70-77 U.S. (Asian ancestry) 95-100 U.S. (Native Americans) 85 Finnish 48 Swiss 12 Swedish 4 Ibos 99 Bantu 90 Fulani 22 Thais 99 13 Biology of Lactose Intolerance • In infants, lactose is broken down by lactase in the small intestine and easily absorbed • If no lactase, the compound travels to the large intestine and is broken down by bacteria causing a build-up of acids and gas • Yogurt and cheese not as much a problem 142/19/2014 8 Biology of Lactose Intolerance • Most mammals – as adults are lactose malabsorbers (lactose intolerant) 15 Reminder: what is a gene? • Segment of DNA that codes for a protein – Structural, enzyme (lactase), horomones • Allele variant forms of the same gene 162/19/2014 9 Genetics of Lactase Persistence • Gene encoding lactase: LCT on chromosome 2 – 3 alleles: LCT*P, LCT*R, LCT*C – LCT*P is dominant (Mendelian trait) • In most people, the genes that produce lactase are turned off around the time of weaning – Mammalian pattern • Two broad groups in which lactase persistence prevails into adulthood (arose independently?) – Populations living in or descended from Northern Europe – Nomadic, pastoral populations in Africa and Asia 17 Evolutionary Advantage of Lactase Persistence • Extra calories and nutrients • Lactase persistence might have allowed people to stay alive during times of drought – Those benefiting from the mutation would have been able to drink milk without the risk of diarrhea 182/19/2014 10 Gene/Culture Co-Evolution • The practice of herding and milking animals arose recently in human history 7,000 – 9,000 years ago • As animal domestication and milk consumption co-evolved, those who could digest lactose were at a selective advantage • Cultural environment affects the selection of a genetic trait! • Is this nature or nurture or is this just a silly question? 19 Fulani Any Questions? 202/19/2014 11 Balanced Polymorphism and Heterozygote Advantage Sickle Cell Anemia 21 Sickle Cell 222/19/2014 12 Sickle Cell Disease • Hemoglobin is coded for by the allele HbA • Some individuals have a variant, HbS, which is deleterious in the homozygous state • Heterozygotes are carriers – Sickling only occurs under severe levels of low oxygen 23 Sickle cell is caused by a point mutation 242/19/2014 13 Pleiotropy in action – sickle cell 25 We might expect that this allele would be eliminated because of its lethal effects, yet in some populations the allele frequency reaches 5% - 20%... What explains this phenomenon?? 262/19/2014 14 27 282/19/2014 15 The HbA – HbS polymorphism is maintained by heterozygote advantage 29 Distribution of the sickle cell trait (HbS) Distribution of malaria Mosquitoes: the vector for Plasmodium, the protozoan parasite that causes malaria 302/19/2014 16 Balanced Polymorphism • HbS allele is selected for in tropical environments because the heterozygotes have the highest fitness in malarial environments 31 1011121314-6.11E-16 0.05 0.1 0.15 0.2 0.25 0.3frequency of sickle cell allele deaths/100 Total number of deaths due to malaria & sickle cell Adapted from Mielke et al. 2006 322/19/2014 17 Sickle cell is not restricted to African populations 33 Any Questions? 342/19/2014 18 Human Variation 35 Objectives • Acclimatization, adaptability, and adaptation – Heat and cold (body size and shape) – High altitude – Skin color • The meaning of “the non-existence of biological races” 362/19/2014 19 Human Adaptation • Like all other species with a broad geographic range, humans have geographic variation • Some differences with other species – – Human dispersal from Africa is very recent (125,000 years ago, maybe less) – Due to short time since dispersal, differences among populations are subtle – Humans have technological as well as biological adaptations to the environment 37 Human Adaptation • Adaptations to local environments – – Color of skin, hair, and eyes – Body size and shape – Disease resistance • Sickle-cell anemia and malaria – Gene/culture co-evolution • Lactose tolerance • Alcohol and/or starch digestion 382/19/2014 20 Adaptation • Acclimatization: short-term changes in
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