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Chapter 15

BIOLOGY 1M03 Chapter 15: Chapter 15- Human Genetic Diveresity

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Ben Evans

Chapter 15: Human Genetic Diversity Explaining Human Variation - wide variation among humans o language, fashions, customs, religion, technology, architecture, and other aspects of behavior differ among societies - peoples of the world vary in blood type and the incidence of many genetically transmitted diseases - scientists distinguish two sources of human variation: genetic and environmental - genetic variation: differences between individuals that are caused by genes they inherited from their parents - environmental variation: differences between individuals caused by environmental factors (i.e. climate, habitat, and competing species) on the organisms’ phenotypes. o for humans, culture is an important source of environmental variation - difficult to determine relative importance of genetic and environmental influences for particular phenotypic traits - genetic variation is governed by processes of organic evolution: mutation, drift, recombination, and selection - variation within groups: differences between individuals within a given group of people - variation among groups: differences between entire groups of people Variation in Traits Influenced by Single Genes - can be certain that variation arises from genetic differences between individuals - example: sickle-cell anemia o know people with this disease are homozygous for a gene that codes for one variant of hemoglobin - can prove that traits are controlled by genes at a single genetic locus by showing that their patterns of inheritance conform to Mendel’s principles - if scientists suspect that a trait is controlled by genes at a single genetic locus, they can test this by collecting data on occurrence of trait in families - if inheritance shows close fit with Mendel’s principles, then we can be confident that traits is affected by a single locus - research on genetic basis of a language disorder called specific language impairment (SLI) o trouble learning to speak o run in families, but genetic basis of condition is unclear in most areas o some cases of SLI are caused by a dominant allele at a single genetic locus o because SLI is rare in population as a whole, the Hardy-Weinberg equations tell us that almost all ALI sufferers will be heterozygotes o although pattern in is consistent with idea that SLI is caused by a single dominant gene, it is possible that an environmental factor causes SLI to run in families and that those observed pattern arose by chance 1 o in 1998, Wellcome Trust researchers found that families with SLI is closely linked to a genetic marker (genes whose location in the genome is known) on chromosome 7 ▪ so it seems likely that SLI is controlled by a gene closely linked to this marker o same allele of this gene FOXP2 has undergone strong directional selection ▪ damage to this gene prevents normal development of some psychological machinery necessary for language Causes of Genetic Variation within Groups - mutation can maintain deleterious genes in populations, but only at a low frequency - many diseases caused by recessive gene - why haven’t such deleterious genes been eliminated by natural selection? o natural selection steadily removes such genes, but they are constantly being reintroduced by mutation o because mutation rates for deleterious genes are estimated to be a few mutations per million gametes produced, mutation will introduce new mutants to maintain a constant frequency of the gene ▪ selection-mutation balance - selection can maintain variation within populations if heterozygotes have higher fitness than either of the two homozygotes - when heterozygotes have a higher fitness than either homozygote has, natural selection will maintain a balanced polymorphism, a steady state in which both alleles persist in the population o example: sickle-cell anemia ▪ individuals with genotype AS (A- regular hemoglobin; S- sickle-cell) are immune to malaria - scientists suspect that relatively high frequencies of genes that cause a number of other genetic diseases may also be result of heterozygote advantage - variation may exist because environments have recently changed and genes that were previously beneficial have not yet been eliminated - some genetic disease may be common because symptoms they create have not always been deleterious o example: non-insulin-dependent diabetes (NIDD) ▪ Neel (from University of Michigan) suggested that genes now leading to NIDD were beneficial in the past because they caused a rapid buildup of fat reserves during periods of plenty- fat reserves that would help people survive periods of famine in harsh environments Causes of Genetic Variation - selection that favours different genes in different environments creates and maintains variation among groups - example: hemoglobin o hemoglobin S is most common in tropical Africa, around Mediterranean Sea, and in southern India o generally, hemoglobin S is prevalent where malaria is common 2 o hemoglobin A is prevalent where this form of malaria is absent - example: lactose o ability to digest lactose as an adult is controlled by a single dominant gene labeled LTC*P ▪ people who have one copy of this continue to synthesize lactase as adults o those who are homozygous for the alternative allele, labeleLTC*R, do not o ability to digest lactose might be common in northern Europe because it helps prevent vitamin D deficiency (since northern Europe has long, dark, cloudy winters, where the sun cannot provide enough sunlight for vitamin D to be synthesized) o Hirschhorn and colleagues (Harvard) have identified 101 single nucleotide polymorphisms (SNPs) in lactase gene surrounding noncoding DNA ▪ these sites vary among populations ▪ if you identify base at one site for an individual, then you can predict base that the same individual will have at the other 100 sites ▪ this is the pattern we would expect to find if there had been recent strong selection on lactase gene because selection will tend to increase frequency of both beneficial DNA segment and all of the closely linked DNA, even if the latter are not directly beneficial - genetic drift creates variation among isolated populations - genetic drift causes random changes in gene frequencies - because genetic drift occurs more rapidly in small populations than in large ones,small populations will diverge from one another faster htan large ones - founder effect: genetic drift caused by expansion of small founding population - genetic drift can also explain why certain genetic diseases are common in some populations and not others - current patterns of genetic variation reflect history of migration and population growth in human species - some of genetic variation among human groups reflects history of peoples of Earth - example: mitochondrial DNA o indicates human species underwent a worldwide population expansion about 100 000 years ago - example: invention of agriculture o led to expansions of farming peoples from Middle East into Europe, from Southeast Asia into Oceania - Sforza (Stanford University) argued that patterns of genetic variation preserve a record of these expansions o human populations expand and move away from each other o as they move farther apart, they become more genetically isolated from one another and accumulate genetic differences - Sforza believes that worldwide pattern of genetic variation results from expansion of modern humans about 100 000 years ago o believe expansion began in Africa 3 Variation in Complex Phenotypic Traits - vast majority of human traits are influenced by many genes, each alone having a relatively
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