Biological Sciences
Mendelian Genetics
Mendelian genetics refers to the principles of inheritance first described by Gregor Mendel in the 19th century. It focuses on the transmission of genetic traits from parents to offspring through discrete units called genes. Mendelian genetics forms the foundation of modern genetics and provides a framework for understanding patterns of inheritance in organisms.
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9 Key excerpts on "Mendelian Genetics"
eBook - PDF- William V Dashek, Marcia Harrison(Authors)
- 2010(Publication Date)
- CRC Press(Publisher)
CHAPTER 10 Mendelian Genetics G.S. Miglani Genetics is the science of heredity and varia- tion. Basteson coined the term genetics in 1905. Heredity is the cause of similarities between individuals. For this reason, broth- ers and sisters with the same parents resemble each other. Variation is the cause of the differences between individuals. This is the reason that brothers and sisters who do resemble each other are still unique indi- viduals. Precisely, genetics is a branch of biology that deals with the mechanisms responsible for transmission of biological properties from one generation to the next. Thus, genetics deals with understanding the nature, molecular structure, organization, biological function, regulation, and manipulation of hereditary particles called genes, which play a central role in life pro- cesses. It also covers the development of ways and means to use the knowledge of genetics for the welfare of mankind. There are certain well-known facts about life: (a) Life comes only from preexist- ing life; (b) Life exists in diverse forms: bacteria, fungi, plants, and animals; (c) Like begets like, e.g. dogs produce pups and cats produce kittens; (d) Traits, i.e. biological characteristics, show recombination. We generally say that the child has a nose like his father and eyes like his mother; (e) Variation exists in living forms. Mem- bers of a species in a natural population differ from one another; (f) Living things have individuality. Biological individuality has pattern, limitations, and characteristics that depend at least in part on the parents of an individual; (g) Similarities and differ- ences are heritable. The basis of genetics is variation and without variation, there would have been no science of genetics. The principles of genetics apply only to sexually reproducing organisms having biparental parentage, microorganisms, plants and animals, including man.
eBook - PDF- William K. Scott, Marylyn D. Ritchie, William K. Scott, Marylyn D. Ritchie(Authors)
- 2021(Publication Date)
- Wiley-Blackwell(Publisher)
This chapter explores the underpinnings for observational and experimental genetics. Concepts ranging from laws of Mendelian inheritance through molecular and chromosomal aspects of deoxyribonucleic acid (DNA) structure and function are defined. The chapter concludes with clinical examples of the various types of DNA mutation and their implications for human disease. Historical Contributions Segregation and Linkage Analysis In 1865, Gregor Mendel, an Austrian monk, published his findings on the inheritance of a series of traits in the pea plant, including seed texture (round or wrinkled), seed color (yellow or green), and plant height (tall or short). He described three properties of heritable factors that explained his quantified observations of these traits. The first property was the unit inheritance, which is now Kayla Fourzali 1 , Abigail Deppen 2 , and Elizabeth Heise 3 1 University of Miami Miller School of Medicine, Miami, FL, USA 2 InformedDNA, St Petersburg, FL, USA 3 Clinical Genetics Program, GeneDX, Inc, Gaithersburg, MD, USA Basic Concepts in Genetics Basic Concepts in Genetics 14 considered the basis for defining the gene. He hypothesized that a factor was transmitted from par-ent to offspring in an unchanged form. Such a factor produced an observable trait. This idea repre-sented a radical departure from the scientific thinking at the time, which suggested parental characteristics were blended in the offspring. Mendel also described the transmission of factors which were controlling observable traits, such as flower color or plant height as a single unit. He proposed that these factors were transmitted independently and with equal frequency to germ cells (egg and sperm) and this observation is referred to as Mendel’s first law, the law of segregation. In his experiments, Mendel crossbred the offspring (the F1 generation) of two phenotypically different, pure-breeding parental strains of peas with one another.
eBook - PDF- Robert Jurmain, Lynn Kilgore, Wenda Trevathan, Eric Bartelink(Authors)
- 2016(Publication Date)
- Cengage Learning EMEA(Publisher)
Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. MENDELIAN INHERITANCE IN HUMANS 79 In 1866, Mendel’s results were published, but the methodology and statistical nature of the research were beyond the thinking of the time, and their significance was overlooked and unappreciated. But by the end of the nineteenth century, several investigators had made important contributions to the field of biology. For example, chromosomes had been discovered and cell division had been explained. In just 34 years, new discoveries and further hypothesis testing had revised many scientists’ views of inheritance. Thus, by 1900, Mendel’s work was generally accepted by biolo-gists, who immediately recognized the importance of his research. Mendel’s research resulted in the discovery of the basic principles of inheritance and further challenged Darwin’s notions of blending inheritance. Unfortunately, he died without knowing the impact his work would have on society. Mendelian Inheritance in Humans Mendelian traits , also called discrete traits , are controlled by alleles at only one genetic locus (or, in some cases, two or more very closely linked loci). The most com-prehensive listing of Mendelian traits in humans is available on the Internet as Online Mendelian Inheritance in Man (OMIM) at www.ncbi.nlm.nih.gov/omim/. Current-ly, this listing includes more than 20,000 human characteristics that are inherited according to Mendelian principles. Although some Mendelian characteristics have readily visible phenotypic expres-sions (such as polydactyly), most don’t. The majority of Mendelian traits are bio-chemical in nature, and many genetic disorders result from harmful alleles inher-ited in Mendelian fashion ( Table 4-1 ). So if it seems like textbooks overly emphasize genetic disease in discussions of Mendelian traits, it’s because so many Mendelian characteristics are the results of harmful alleles.
eBook - PDFReconceiving the Gene
Seymour Benzer's Adventures in Phage Genetics
- Frederic Lawrence Holmes, William C. Summers(Authors)
- 2008(Publication Date)
- Yale University Press(Publisher)
C HAPTER O NE Classical Mendelian Genetics The formation of genetics during the first decades of the twentieth century, following the rediscovery of the long-overlooked paper by Gregor Mendel that provided its fundamental principles, has been the subject of many historical accounts. 1 In this chapter I do not attempt to recapitulate this complex history but only draw attention to certain features of its development that were particularly relevant to what was viewed by mid-century as ‘‘classical genetics.’’ Mendel explained the results of his experiments on the hybridiza-tion of pea plants by assuming the presence in the germ cells of An-lagen that give life to the individuals that display the particular Merk-male by reference to which he differentiated them. When two plants whose Anlagen produced different Merkmale, such as green or yellow seed coats, were mated, the effects of only one of them, which he called the dominant one, were visible in the hybrid progeny. That in the next generation both green- and yellow-coated seeds appeared in defi-nite ratios he attributed to the other, or ‘‘recessive,’’ Anlage having remained unaltered during their association, the two types then segre-gating independently during the formation of the germ cells. 2 At the time there were no structures identified within ordinary or germ cells with which the Anlage could be associated. Mendel’s term Anlage was later translated in the English literature as ‘‘factor,’’ and Merkmale as ‘‘character.’’ Whereas the paired German words were suggestive of the relation between an inner predisposi-tion and an outward sign, the words factor and character lacked these connotations. Because the factors remained abstract entities without known properties of their own, early geneticists often associated them so closely with the ‘‘unit characters’’ they were supposed to produce that some geneticists nearly obliterated the distinction between factor
eBook - PDFThe Mendelian Revolution
The Emergence of Hereditarian Concepts in Modern Science and Society
- Peter J. Bowler(Author)
- 2000(Publication Date)
- The Athlone Press(Publisher)
Thus it was in 190() that three biologists. Carl Correns, Hugo De Vries and E. von Tschermak, independently rediscovered Mendel's laws and came to acknowledge the work of the now-dead pioneer. Although at first resisted by biologists unable to throw off outdated notions, the new approach gained an increasing number of converts and soon established itself as the basis for a new science of heredity. Soon the work of T. H. Morgan and his school, using the fruit fly, Drosophila, showed that the behaviour of the chromo-somes during reproduction corresponded exactly to the transmis-sion of characters according to Mendel's laws. Classical genetics 4 The Mendelian Revolution could now exploit the concept of the gene as a discrete material unit on the chromosome, coding for a particular character that could be transmitted from parent to offspring, isolated from all external influence. The new theory resolved the difficulties inherent in Darwin's original formulation of natural selection, allowing the 'genetical theory of natural selection' to become the dominant paradigm in evolutionary biology. Genetics ushered in a new era in the science of animal and plant breeding, and in the identification of hereditary defects in humans. Once the complexities of the transmission process were under-stood, the way was clear for the emergence of molecular biology, a second generation science that would explain the biochemical and physiological processes by which the hereditary information was coded and then unfolded into living tissue. Recognition of the 'double helix' structure of DNA (the chemical of which the genetic material is composed) by James Watson and Francis Crick in 1953 opened the way to yet greater understanding and control of the process of inheritance (Watson, 1968; Olby, 1974). The key element in this orthodox story is the notion of discovery. The account is based on the assumption that genetics -like all sciences -reveals true information about the real world.
eBook - PDF- Arnold W. Ravin, Alvin Nason(Authors)
- 2013(Publication Date)
- Academic Press(Publisher)
CHAPTER I THE LEGACY OF CLASSICAL GENETICS Biologists generally agree about the year of the birth of the science of genetics. Prior to the work of Gregor Mendel there existed no theoretical insight into the problems of inheritance, no insight, at least, that could lend coherence to the then existing experimental observations about animal and plant breeding nor direction to further research. In the middle of the nineteenth century, Mendel was crossing cer-tain varieties of the pea plant in his monastery garden and observing remarkable and reproducible patterns of trans-mission from one plant generation to the next. He took the admirable step of furnishing an abstract, but experimentally verifiable explanation of his findings, and his theoretical contribution must be credited as having launched the new biological discipline concerned with the transmission of potentialities from parents to offspring. It was not until 1900, however, that biologists recognized, in what they did, the significance of Mendel's contribution. The reasons for this delay should prove a fertile subject for students of the history of ideas, but we are concerned here with the dating of an important event, the birth of genetics. Since 1900 marks the year when Mendel's ideas were put to further tests by biologists, which resulted in the rapid development 1 2 I. The Legacy of Classical Genetics of an enormously fruitful science (a development not yet come to term), genetics is recorded as having been born in that year. It is, consequently, a mere baby of a science, barely sixty-five years old. It may be presumptuous to distinguish in so young a science a classical and a modern period. Yet the very speed with which genetic knowledge has expanded may account for the precocity with which fundamentally new ways of viewing things have succeeded each other in genet-ics. In any event, a significant change does seem to have come about in the way geneticists pose and attack questions.
eBook - PDF- Robert Jurmain, Lynn Kilgore, Wenda Trevathan, Russell Ciochon(Authors)
- 2017(Publication Date)
- Cengage Learning EMEA(Publisher)
random assortment The chance distribution of chromosomes to daughter cells during meiosis. Along with recombi-nation, random assortment is an important source of genetic variation (but not new alleles). Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-300 MENDELIAN INHERITANCE IN HUMANS 87 In 1866, Mendel’s results were published, but the methodology and statistical nature of the research were beyond the thinking of the time, and their significance was overlooked and unappreciated. But by the end of the nineteenth century, several investigators had made important contributions to the field of biology. For exam-ple, chromosomes had been discovered and cell division had been explained. In just 34 years, new discoveries and further hypothesis testing had revised many scien-tists’ views of inheritance. Thus, by 1900, Mendel’s work was generally accepted by biologists, who immediately recognized the importance of his research. Mendelian Inheritance in Humans Mendelian traits , also called discrete traits , are controlled by alleles at only one genetic locus (or, in some cases, two or more very closely linked loci). The most comprehensive listing of Mendelian traits in humans is available on the Internet. Online Mendelian Inheritance in Man (www.ncbi.nlm.nih.gov/omim/) currently lists more than 21,000 human characteristics that are inherited according to Mendelian principles. F 1 Generation Genotype Genotypes Pure-breeding short plant with green seeds (Recessive traits) × TTYY Pure-breeding tall plant with yellow seeds (Dominant traits) 3 / 16 short with yellow seeds 9 / 16 tall with yellow seeds 1 / 16 short with green seeds TTYY TTYy TtYY TtYy ttyy 3 / 16 tall with green seeds TTyy Ttyy ttyY ttYy All tall plants with yellow seeds Genotype Phenotype Phenotype Phenotype ttyy TtYy F 2 Generation ◂ Figure 4-6 Results of a cross when two traits (height and seed color) are considered simultaneously.
eBook - PDF- Kostas Kampourakis(Author)
- 2021(Publication Date)
- Cambridge University Press(Publisher)
2 The Origin and Evolution of the Gene Concept Mendel and the Gene Concept Perhaps you were taught at school that genetics began with Gregor Mendel. Because of his experiments with peas, Mendel is considered to be a pioneer of genetics and the person who discovered the laws of heredity. According to the model of “Mendelian inheritance,” things are rather simple and straight- forward with inherited characteristics. Some alleles are dominant – that is, they impose their effects on other alleles that are recessive. An individual who carries two recessive alleles exhibits the respective “recessive” characteristic, whereas a single dominant allele is sufficient for the “dominant” version of the characteristic to appear. In this sense, particular genes determine particu- lar characteristics (e.g., seed color in peas), and particular alleles of those genes determine particular versions of the respective characteristics. Mendel, the story goes, discovered that characteristics are controlled by hereditary factors, the inheritance of which follows two laws: the law of segregation and the law of independent assortment. In the first case, when two plants that differ in one characteristic, such as having seeds that are either round or wrinkled, are crossed, their offspring (generation 1) resemble one of the two parents (in this case, they have round seeds). In generation 2 (the offspring of the offspring) there is a constant ratio 3:1 between the round and the wrinkled shapes (Figure 2.1). Round shape is controlled by factor R, which is dominant, whereas wrinkled shape is con- trolled by factor r, which is recessive. Dominant and recessive practically Figure 2.1 A cross between two plants that differ in the shape of seeds (round or wrinkled). Plants with round seeds have factors RR or Rr, whereas plants with wrinkled seeds have factors rr. The “wrinkled” characteristic “disappears” in generation 1 and “reappears” in generation 2 (all possible combinations of gametes are made).
eBook - PDF- D. Peter Snustad, Michael J. Simmons(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
In addition, humans—unlike experimental organisms—do not Mendelian Principles in Human Genetics Mendel’s principles can be applied to study the inheritance of traits in humans. Mendelian Principles in Human Genetics 53 produce many progeny, making it difficult to discern Mendelian ratios, and humans are not maintained and observed in a controlled environment. For these and other reasons, human genetic analysis has been a difficult endeavor. Nonetheless, the drive to understand human heredity has been very strong, and today, despite all the obstacles, we have learned about thousands of human genes. Table 3.3 lists some of the conditions they control. We will discuss many of these conditions in later chapters of this book. PEDIGREES Pedigrees are diagrams that show the relationships among the members of a family (◾ Figure 3.13a). It is customary to represent males as squares and females as circles. A horizontal line connecting a circle and a square represents a mating. The offspring of the mating are shown beneath the mates, starting with the first born at the left and proceeding through the birth order to the right. Individuals that have a genetic condition are indicated by coloring or shading. The generations in a pedigree are usu- ally denoted by Roman numerals, and particular individuals within a generation are referred to by Arabic numerals following the Roman numeral. Traits caused by dominant alleles are the easiest to identify. Usually, every individual who carries the dominant allele manifests the trait, making it possible to trace the trans- mission of the dominant allele through the pedigree (◾ Figure 3.13b). Every affected individual is expected to have at least one affected parent, unless, of course, the dominant allele has just appeared in the family as a result of a new mutation—a change in the gene itself.
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