Genetic Variation

12 Key excerpts on "Genetic Variation"

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  Biology Today

  An Issues Approach

  • Eli Minkoff, Pamela Baker(Authors)
  • 2003(Publication Date)
  • Garland Science

    (Publisher)

  There Is Biological Variation Both Within and Between Human Populations All genetic traits in humans and other species vary considerably from one individual to another. Some of this variation consists of different alleles at each gene locus; other variation results from the interaction of genotypes with the environment. The simplest type of variation governs traits such as those discussed in Chapter 3 (pp. 75–77), in which an enzyme may either be functional or nonfunctional. The inheritance of these traits follows the patterns described in Chapter 2, which you may want to review at this time. In particular, be sure that you understand the meaning of dominant and recessive alleles and of homozygous and het-erozygous genotypes. Many other traits, as we saw in Chapter 3, have a more complex genetic basis. In this section we examine how biological variation is described. Continuous and discontinuous variation within populations Many human traits vary over a range of values, with all intermediate values being possible; such variation is called continuous variation . Human Variation 7 Figure 7.1 Continuous variation in a single population: all intermediate values are possible. There Is Biological Variation Both Within and Between Human Populations 205 Continuously variable traits, such as height, can often be measured in an individual and expressed as a numerical value. Other traits that vary con-tinuously, such as hair curliness or skin color, are seldom expressed numerically, although theoretically they could be. Continuous variation can result from the cumulative effects of multi-ple genes, each of which by itself contributes a small effect. Dozens of known genes, perhaps even hundreds, influence height in one direction or another.

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  Textbook of Biodiversity

  • K V Krishnamurthy(Author)
  • 2003(Publication Date)
  • CRC Press

    (Publisher)

  In organisms that exhibit a sexual mode of reproduction, the resultant Genetic Variations can spread through the population by recombination. In inbreeding members of a population, the pool of Genetic Variation already present is acted upon by selection. Differential survival results in changes in the frequency of genes within this pool, in tum resulting in population evolution. Therefore, the main significance of Genetic Variation in a population is that it enables natural evolutionary changes to take place since the rate of these changes is proportional to the amount of genetic diversity available. Genetic Diversity is a resource for the species' own survival and future evolution; it also promotes selective breeding (see Groombridge 1992). In addition, Genetic Variation also confers fitness advantages upon the members of a population. Another reason why it should be considered a resource is that many genes are potential sources for improving the productivity of other members of a population or of a species. For example, genes from wild plants have become very important today for improving domesticated taxa (for more details, see Chapters 4 and 10). The global pool of genetic diversity represents all the information pertinent to all biological structures, functions and processes on this planet. In other words, every process and every pattern is encoded in a genetic 'library' of unimaginable global extent. But only a negligible fraction (often less than 1 %) of the genetic material of higher organisms is expressed in the form and functions of organisms (see Groombridge 1992). This means that much of the DNA in each cell is not coded (Thomas 1992). The role of such 'silent' DNA in the expression of variations still remains unclear. Again, of the estimated 10 9 different 11 genes in the world biota (of which about lOS are distributed in higher organisms) not one has been found to duplicate the contribution of another to the overall genetic diversity.

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  Thompson & Thompson Genetics in Medicine E-Book

  Thompson & Thompson Genetics in Medicine E-Book

  • Robert L. Nussbaum, Roderick R. McInnes, Huntington F Willard(Authors)
  • 2015(Publication Date)
  • Elsevier

    (Publisher)

  This chapter is one of several in which we explore the nature of genetically determined differences among individuals. The sequence of nuclear DNA is approximately 99.5% identical between any two unrelated humans. Yet it is precisely the small fraction of DNA sequence difference among individuals that is responsible for the genetically determined variability that is evident both in one's daily existence and in clinical medicine. Many DNA sequence differences have little or no effect on outward appearance, whereas other differences are directly responsible for causing disease. Between these two extremes is the variation responsible for genetically determined variability in anatomy, physiology, dietary intolerances, susceptibility to infection, predisposition to cancer, therapeutic responses or adverse reactions to medications, and perhaps even variability in various personality traits, athletic aptitude, and artistic talent.

  One of the important concepts of human and medical genetics is that diseases with a clearly inherited component are only the most obvious and often the most extreme manifestation of genetic differences, one end of a continuum of variation that extends from rare deleterious variants that cause illness, through more common variants that can increase susceptibility to disease, to the most common variation in the population that is of uncertain relevance with respect to disease.

  The Nature of Genetic Variation

  As described in Chapter 2 , a segment of DNA occupying a particular position or location on a chromosome is a locus (plural loci ). A locus may be large, such as a segment of DNA that contains many genes, such as the major histocompatibility complex locus involved in the response of the immune system to foreign substances; it may be a single gene, such as the β-globin locus we introduced in Chapter 3 ; or it may even be just a single base in the genome, as in the case of a single nucleotide variant (see Fig. 2-6 and later in this chapter). Alternative versions of the DNA sequence at a locus are called alleles . For many genes, there is a single prevailing allele, usually present in more than half of the individuals in a population, that geneticists call the wild-type or common allele. (In lay parlance, this is sometimes referred to as the “normal” allele. However, because Genetic Variation is itself very much “normal,” the existence of different alleles in “normal” individuals is commonplace. Thus one should avoid using “normal” to designate the most common allele.) The other versions of the gene are variant (or mutant ) alleles that differ from the wild-type allele because of the presence of a mutation, a permanent change in the nucleotide sequence or arrangement of DNA. Note that the terms mutation and mutant

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  Genomes, Evolution, and Culture

  Past, Present, and Future of Humankind

  • Rene J. Herrera, Ralph Garcia-Bertrand, Francisco M. Salzano(Authors)
  • 2016(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Chapter 4 Genetic Variability

  At bottom every man knows well enough that he is a unique being, only once on this earth; and by no extraordinary chance will such a marvelously picturesque piece of diversity in unity as he is, ever be put together a second time.

  —Friedrich Nietzsche [1]

  Summary

  A number of forces act at the population level to shape gene pools. Mutations, the raw material of evolution, are the sources of variability that are then acted on by mechanisms that alter the abundance of genetic variants. These forces include genetic drift, selection, bottleneck events, founder effects, isolation, nonrandom mating, and inbreeding. Mutations, for the most part, occur at random throughout the genome and happen spontaneously or can be induced by chemicals or radiation. DNA areas rich in repetitive sequences, short tandem repeats (STRs), and GpC dinucleotides experience elevated mutation rates. Some mutations are subject to natural selection, yet others are selectively neutral or almost neutral. Mutations that are under selection pressure usually are retained in genomes as a result of balanced polymorphisms in which positive and negative selection pressures reach a state of dynamic equilibrium. In the state of balanced polymorphisms, heterozygotes have a higher fitness than both homozygotes, and directional selection occurs when one homozygote is favored (positive selection) compared with the other (negative selection). Sickle-cell anemia is a classical example. Neutral mutations are particularly useful in studies of genetic variability since they represent markers that reflect true ancestry more faithfully. The phenotype not only is dictated by our DNA but always has an environmental component. Mutations occur in somatic or germline tissues with different consequences, the former being a contributor to the genesis of cancer and the latter as a source of evolutionary change. The environment is never constant and since selection pressure affects allelic frequencies of many genes, the frequencies of specific genetic variants fluctuate. These selection-driven alterations in allelic frequencies may obscure the true phylogenetic relationships among groups of organisms.

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  Biochemistry of Insects

  • Morris Rockstein(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  14 Chemical Genetics and Evolution FRANCISCO J. AYALA I. Introduction 579 II. Genetic Variation in Natural Populations 580 A. Genetic Variation and Rate of Evolution 580 B. Early Evidence of Genetic Variation 581 C. The Problem of How to Measure Genetic Variation 583 D. Protein Variation as Genetic Variation 585 E. Statistics for Measuring Genetic Variation 589 F. Estimates of Genetic Variation 590 III. Genetic Differentiation during Speciation 595 A. Anagenesis and Cladogenesis 595 B. Geographic Speciation . . . . 597 C. A Paradigm of the Process of Speciation 598 D. Biochemical Differentiation in the Drosophila willistoni Group . 601 IV. Reconstruction of Evolutionary History 604 A. Conceptual Basis of Comparative Studies 604 B. Electrophoretic Phylogenies 605 C. Immunology and Protein Sequencing 607 D. DNA Hybridization and Sequencing 610 E. The Molecular Clock of Evolution 614 General References 615 References for Advanced Students and Research Scientists . . 616 I. INTRODUCTION Evolution, consisting of changes in the genetic constitution of popula-tions, can only occur if there is Genetic Variation, so that alternative genetic variants increase or decrease in frequency over the generations. Thus a fundamental question in evolutionary genetics is how much genetic varia-579 580 Francisco J. Ayala tion exists in natural populations. The amount of Genetic Variation in a population measures its evolutionary potential. Section II of this chapter deals with the methods and the results of studies aimed at ascertaining the amount of variation in natural populations, especially of insects. Early studies failed to provide quantitatively precise estimates of Genetic Variation; however, these have become possible in recent years through the application of biochemical concepts and biochemical methods, particularly gel elec-trophoresis. Evolutionary genetics is also concerned with how much genetic change takes place in evolution.

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  Medical Genetics E-Book

  Medical Genetics E-Book

  • Lynn B. Jorde, John C. Carey, Michael J. Bamshad(Authors)
  • 2019(Publication Date)
  • Elsevier

    (Publisher)

  3

  Genetic Variation

  Its Origin and Detection

  Humans display a substantial amount of Genetic Variation. This is reflected in traits such as height, blood pressure, and skin color. Included in the spectrum of Genetic Variation are disease states, such as cystic fibrosis or type 1 neurofibromatosis (see Chapter 4 ). This aspect of Genetic Variation is the focus of medical genetics.

  All Genetic Variation originates from the process known as mutation, which is defined as a change in DNA sequence. Mutations can affect either germline cells (cells that produce gametes) or somatic cells (all cells other than germline cells). Mutations in somatic cells can lead to cancer and are thus of significant concern. However, this chapter is focused primarily on germline mutations, because they can be transmitted from one generation to the next.

  As a result of mutations, a gene can differ among individuals in terms of its DNA sequence. The differing sequences are referred to as alleles. A gene’s location on a chromosome is termed a locus (from the Latin word for “place”). For example, it might be said that a person has a certain allele at the β-globin locus on chromosome 11. If a person has the same allele on both members of a chromosome pair, they are said to be a homozygote. If the alleles differ in DNA sequence, they are a heterozygote. The combination of alleles that is present at a given locus is termed the genotype.

  In human genetics, the term mutation has often been reserved for DNA sequence changes that cause genetic diseases and are consequently relatively rare, with a population frequency less than 1%. DNA sequence variants that are more common in populations are conventionally described as polymorphisms (“many forms,” describing multiple alleles at a locus). Loci (plural of locus) that contain multiple alleles are termed polymorphic. Nowadays, however, alleles that have a frequency less than 1% are sometimes called polymorphisms as well. In addition, many common polymorphisms are now known to influence the risks for complex, common diseases such as diabetes and heart disease (see Chapter 12

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  Introduction to Physical Anthropology

  • Robert Jurmain, Lynn Kilgore, Wenda Trevathan, Russell Ciochon(Authors)
  • 2017(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  But in the last 60 years or so, the emphasis has shifted to examining the differences in allele frequencies (and, more basically, DNA differ-ences) within and between populations, as well as considering the adaptive signifi-cance of phenotypic and genotypic variation. This shift in focus occurred partly because of the Modern Synthesis in biology. But now, armed with genome data sets for populations, biologists have an unprecedented opportunity to study and explain human variation and the role that evolutionary factors have played in producing it (Pritchard, 2010). In the twenty-first century, the application of evolutionary principles to the study of modern human variation has replaced the superficial nineteenth-century view of race based solely on observed phenotype . Additionally, the genetic emphasis has dispelled previously held misconceptions that races are fixed biological enti-ties that don’t change over time and are composed of individuals who all conform to a particular type . Clearly, there are visible phenotypic differences between humans, and some of these roughly correspond to particular geographical locations. But we need to ask if there’s any adaptive significance attached to these differences. Is genetic Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-300 THE CONCEPT OF RACE 417 drift a factor? What is the degree of underlying Genetic Variation that influences phenotypic variation? What influence has culture had in the past? These questions place considerations of human variation within a contemporary evolutionary, bio-cultural framework. Although, as a discipline, physical anthropology is rooted in attempts to explain human diversity, no contemporary scholar subscribes to pre–Modern Synthesis concepts of races (human or nonhuman) as fixed biological entities.

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  The Molecular Basis of Plant Genetic Diversity

  • Mahmut Caliskan(Author)
  • 2012(Publication Date)
  • IntechOpen

    (Publisher)

  Part 1 Genetic Diversity in Plant Populations 1 Genomics Meets Biodiversity: Advances in Molecular Marker Development and Their Applications in Plant Genetic Diversity Assessment Péter Poczai 1,2 , Ildikó Varga 2 , Neil E. Bell 1,3 and Jaakko Hyvönen 1 1 Plant Biology, University of Helsinki, Helsinki 2 Department of Plant Science and Biotechnology, Georgikon Faculty University of Pannonia, Keszthely 3 Botanical Museum, University of Helsinki, Helsinki 1,3 Finland 2 Hungary 1. Introduction Genetic diversity is the fundamental source of biodiversity – the total number of genetic characters contributing to variation within species. In other words it is the measure that quantifies the variation found within a population of a given species. Genetic diversity among individuals reflects the presence of different alleles in the gene pool, and hence different genotypes within populations. Genetic diversity should be distinguished from genetic variability, which describes the tendency of genetic traits found within populations to vary (Laikre et al., 2009). Since the beginning of the 20 th century, the study of genetic diversity has been the major focus of core evolutionary biology. The theoretical metrics developed, such as genetic variance and heritability (Fisher, 1930; Wright, 1931), provided the quantitative standards necessary for the evolutionary synthesis. Further research has focused on the origin of genetic diversity, its maintenance and its role in evolution. Simple questions such as “who breeds with whom” initiated studies on the relatedness of populations. These investigations led to the formation of metapopulation theory, where a group of spatially separated populations of the same species interact at some level and form a coherent larger group (Hanski, 1998). The discovery of spatial structure in populations was a key element in the early concepts and models of population ecology, genetics and adaptive evolution (Wright, 1931; Andrewartha & Birch, 1954).

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  Human Evolutionary Genetics

  • Mark Jobling, Edward Hollox, Toomas Kivisild, Chris Tyler-Smith(Authors)
  • 2013(Publication Date)
  • Garland Science

    (Publisher)

  Section 2

  How do we interpret Genetic Variation?

  I nsights into the human past can be generated by analyzing modern genetic diversity because Genetic Variation has not arisen independently in each individual, but has a shared history that has been shaped by different processes, such as natural selection and population expansion. This section introduces the fundamental concepts of population genetics, molecular evolution, and phylogenetics that allow us to infer the history of Genetic Variation, and the processes responsible for the observed pattern of Genetic Variation. These concepts are central to the rest of this book.

  From Tishkoff et al. (2009) Science 324,1035. With permission from AAAS.

  CHAPTER 5 PROCESSES SHAPING DIVERSITY

  This chapter demonstrates that genetic diversity within a species is shaped by a number of different processes. These include mutation and recombination, which generate new Genetic Variation; genetic drift, which removes variation; and selection, which shapes preexisting variation. The interplay of these different forces is complex and requires us to develop mathematical models that describe simplified but fundamental features of how populations and molecules change over time.

  CHAPTER 6 MAKING INFERENCES FROM DIVERSITY

  The knowledge gained from modeling these different processes allows us to extract information about the past from genetic diversity data. In this chapter we describe methods of analyzing genetic diversity that allow us to investigate the role that each of these processes has played in shaping variation. Case studies to illustrate the application of these inferential methods are found in subsequent chapters and readers are referred forward to the relevant sections. Similarly, in the later chapters readers are directed back to this section for methodological detail. Some may wish to refer to the contents of Chapters 5 and 6

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  Human Variation

  Races, Types, and Ethnic Groups

  • Stephen Molnar(Author)
  • 2015(Publication Date)
  • Routledge

    (Publisher)

  chapter

  2

  The Biological Basisfor Human Variation        

  The journey from the discovery of particulate inheritance to the definition of gene structure has been a long one, occupying most of the last century. From simple observations of inheritance between generations of easily perceived traits to descriptions of the chemical nature of the gene (DNA), we have witnessed an explosion of biochemical data on cell function and control of metabolic processes. Over this time period, assisted by evermore sophisticated techniques, we are able to view inheritance at several levels: trait transmission between generations, gene and gene combinations, and the molecular or biochemical structure.

  With our new-found knowledge it is all too easy to forget or even to ignore some of the key landmarks along the road leading to an understanding of the biological basis for human variation. Many of these landmarks are fundamental for comparisons of population diversity, and it is best to refresh our memory from time to time if we are to appreciate the new molecular genetic discoveries and their meanings for human adaptations to our changing environments.

  First, recall that humans share similar modes of reproduction with most other mammals, and inheritance mechanisms are the same—the combination of certain materials from the germ cells of male and female parents to produce a fertilized egg. These mechanisms of inheritance are the source of much of the vast diversity seen in the biological world. For many centuries, natural scientists had sought to comprehend and explain this diversity in the transmission of traits between generations. Explanations varied from a description of a “blending” of parental bloodlines, favored by animal husbandry, to a theory of “preformism,” the idea that the individual, in miniature form, existed in either the ovum or sperm awaiting stimulation by fertilization to begin its development. None of these explanations could account for the ranges of individual similarities or differences among offspring and their parents.

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  Biosocial Interactions in Modernisation

  • Robert Cliquet(Author)
  • 2014(Publication Date)
  • Masarykova univerzita

    (Publisher)

  At the separation of the combinations between the chromosomes, 80 23 possible types of gametes can be produced (Bennett, 1979). Current estimates indicate that all humans are approximately 99.6 to 99.8 percent identical at the nucleotide sequence level. Within the remaining 0.2−0.4 percent genetic material, approximately 10 million DNA variants can potentially occur in different combinations. This represents a very small fraction of the total genome, but is vastly more than enough variation to ensure individual uniqueness at the DNA level (Tishkoff and Kidd, 2004). With the exception of monozygotic (identical) twins, where the segregation-recombination-mechanism is by-passed, no two individuals have the same genome. The individual has a unique genetic identity (Harris, 2006). 3 Diploid cell: somatic cell containing two complete sets of chromosomes, one set derived from each parent. 4 A chromatid is one of the two identical copies of a duplicating chromosome during the process of cell division. INDIVIDUAL VARIATION AND INDIVIDUALISM /59 Evolutionary mechanisms and individual variation Biological differences between individuals within populations are largely due to the various evolutionary mechanisms which today constitute the so-called modern synthetic evolutionary theory which took shape in the 1930’s (Chetverikov, 1926; Fisher, 1930; Haldane, 1932; Wright, 1931): mutations, natural selection, genetic migration, genetic drift, and non-random partner choice. They allow for explaining not only intergenerational changes in the genetic composition and structure of populations – i.e. evolution – but also the within-generational variation between individuals. The Hardy-Weinberg law The Genetic Variation that exists between individuals within a population cannot always be directly deduced from the observation of phenotypic distribution, in other words, from the way biological features are manifested.

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  Biology Trending

  A Contemporary Issues Approach

  • Eli Minkoff, Jennifer K. Hood-DeGrenier(Authors)
  • 2023(Publication Date)
  • CRC Press

    (Publisher)

  gene pools , of populations, making it progressively more difficult to identify any significant differences between populations.

  7.1.4 The study of human variation

  All studies of human variation carry the risk of being misused or misinterpreted by racists. Nevertheless, there are many good reasons for studying human variation, and this study serves as the basis for the entire field of “human factors engineering.” To take a simple example, the design of a passenger compartment (for automobiles, aircraft, etc.) must accommodate a certain range in the size, sitting height, arm length, and other dimensions of its possible occupants. These and other accommodations must take into account the total range of human variation, including all races and both sexes. In airline cockpits and similar enclosures, controls should be both visible and reachable by persons of different sizes. Moreover, these features are often matters of safety as well as comfort. Vehicle seat belts and airbags, sports equipment, surgical equipment, wheelchairs and similar aids, boots, helmets, kitchen counters, telephone receivers, gas masks, toilets, and doorways all need to accommodate the range of dimensions of the human body. Variation in other human characteristics (breathing rates, sweating) must also be considered in the design of space suits, diving equipment, respiratory equipment for fighting fires, or protective clothing for other situations. Most of the variation relevant for human factors engineering is found within each population group, including variation by age and sex; variation between human populations is generally minor by comparison.

  A further reason for studying Genetic Variation among human populations is that it can help us understand evolution. Population genetics has helped us recognize geographic patterns of disease resulting from natural selection acting on human populations. Studies of this kind can also help us to reconstruct the past history of particular human populations or of the human species as a whole. In the following sections of this chapter, we examine some of these studies.

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