Biological Sciences
Mendel's Law of Segregation
Mendel's Law of Segregation states that during the formation of gametes, the two alleles for a gene segregate from each other so that each gamete carries only one allele for each gene. This means that offspring inherit one allele from each parent, leading to genetic diversity in the population.
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10 Key excerpts on "Mendel's Law of Segregation"
eBook - PDFSocial Mendelism
Genetics and the Politics of Race in Germany, 1900–1948
- Amir Teicher(Author)
- 2020(Publication Date)
- Cambridge University Press(Publisher)
In one of them, there are only two Mendel- ian laws. The first law is the Law of Segregation, which states that in sexually reproducing organisms, the hereditary factors received from both parents for any given trait do not blend into each other but remain separate. When the organism reproduces, it passes on to each of its descendants, with equal probability, either the factor it received from its father or the one from its mother. As a result, when conducting controlled hybridization experiments, it is possible (in principle) to com- pute the chances that a certain trait will appear among the progeny. The second law, the Law of Independent Assortment, states that factors for different traits are inherited independently of each other. For example, the color of the pea and its height are different traits; therefore, their respective hereditary factors are mutually independent. 22 In the alternative version, there exist three Mendelian laws. The first is the Law of Uniformity (Uniformitätsregel), which states that all the 21 This has allowed for considerable scope of interpretation regarding Mendel’s real objectives in constructing his experiments as well as in compiling his paper, and consequently also regarding the essence of Mendel’s work in Mendel’s own eyes. The scholarly debate on these matters has yielded a range of thought-provoking studies and unorthodox historical interpretations. For example, see Robert Olby, “Mendel no Mendelian?” History of Science 17 (1979): 53–72; Alain F. Corcos and Floyd V. Monaghan, “The Real Objective of Mendel’s Paper,” Biology and Philosophy 5 (1990): 267–292; Raphael Falk and Sahotra Sarkar, “The Real Objective of Mendel’s Paper: A Response to Monaghan and Corcos,” Biology and Philosophy 6 (1991): 447–451; Floyd V.
eBook - PDF- Julianne Zedalis, John Eggebrecht(Authors)
- 2018(Publication Date)
- Openstax(Publisher)
Since Mendel’s experiments with pea plants, other researchers have found that the law of dominance does not always hold true. Instead, several different patterns of inheritance have been found to exist. Chapter 12 | Mendel's Experiments and Heredity 495 Figure 12.15 The child in the photo expresses albinism, a recessive trait. Equal Segregation of Alleles Observing that true-breeding pea plants with contrasting traits gave rise to F 1 generations that all expressed the dominant trait and F 2 generations that expressed the dominant and recessive traits in a 3:1 ratio, Mendel proposed the law of segregation. This law states that paired unit factors (genes) must segregate equally into gametes such that offspring have an equal likelihood of inheriting either factor. For the F 2 generation of a monohybrid cross, the following three possible combinations of genotypes could result: homozygous dominant, heterozygous, or homozygous recessive. Because heterozygotes could arise from two different pathways (receiving one dominant and one recessive allele from either parent), and because heterozygotes and homozygous dominant individuals are phenotypically identical, the law supports Mendel’s observed 3:1 phenotypic ratio. The equal segregation of alleles is the reason we can apply the Punnett square to accurately predict the offspring of parents with known genotypes. The physical basis of Mendel’s law of segregation is the first division of meiosis, in which the homologous chromosomes with their different versions of each gene are segregated into daughter nuclei. The role of the meiotic segregation of chromosomes in sexual reproduction was not understood by the scientific community during Mendel’s lifetime. Independent Assortment Mendel’s law of independent assortment states that genes do not influence each other with regard to the sorting of alleles into gametes, and every possible combination of alleles for every gene is equally likely to occur.
eBook - PDFDiscoveries In Plant Biology (Volume I)
(Volume I)
- Shang-fa Yang, Shain-dow Kung(Authors)
- 1998(Publication Date)
- World Scientific(Publisher)
Chapter 11 The Discovery of Menders Laws George H. Liang Dept. of Agronomy, Kansas State Univ., Manhattan, KS 66506, USA D. Z. Skinner USDA/ARS, Kansas State Univ., Manhattan, KS 66506, USA YiSun Biotechnology Center, Shanxi Academy of Agricultural Sciences, Taiyuan, Shanxi, China E. L. Sorensen USDA/ARS, Kansas State Univ., Manhattan, KS 66506, USA J. H. Guo Division of Biology, Kansas State Univ., Manhattan, KS 66506, USA ABSTRACT Gregor Mendel published his famous research results, using garden pea 130 years ago. His experiments were classical and have a molecular basis. Fundamental laws that he established are: (1) law of segregation, in which the two members of a gene pair segregate from each other during formation of sex cells; half the sex cells carry one member of the pair and the other half carry the other member of the pair, and (2) law of independent assortment, where different segregating gene pairs behave independently. His laws appear to apply to all eukaryotic organisms and form a basis for further development and extension of complex genetic principles. Mendel's seemingly simple experiments have complex controlling mechanisms involving biochemistry and physiology. For example, Mendel studied seed shape, round versus wrinkled, which segregates as a single gene. Wrinkled seeds (rr) lack an isoform of the starch branching enzyme (SBE I). Round seeds, in homozygous (RR) or heterozygous (Rr) state, produce functional SBE I. In rr plants, the SBE I gene has a 0.8 kb insertion, similar to the Ac/Ds family of transposable elements from maize, resulting in an aberrant SBE I transcript. 145 146 George H. Liang, D. Z. Skinner, Yi Sun, E. L. Sorenson and J. H. Guo Regarding the height character, tall plants contain more gibberellin 1 (GA 1) than dwarfs and the allele Le synthesizes an enzyme that 3 p-hydroxylates GA 20 to produce GA 1. In the absence of this enzyme, GA 1 is not produced and the plants were dwarfs.
eBook - PDF- Tomar, Arun Kumar(Authors)
- 2021(Publication Date)
- Daya Publishing House(Publisher)
Thus, there are two kinds of equally frequent pollen ( R and r ) and two kinds of equally frequent eggs ( R and r ). Therefore, members of a given pair of gene ( R and r ) segregate (separate) from each other during gamete formation, into equal numbers of gametes. The recessive gene ( r ) expresses itself in F 2 generation. Mendel called to this reappearance of the parental recessive character (wrinkle) as the segregation (separation) of the members of a given pair of gene ( R and r ). It is known as the principle of segregation. The law of segregation states that genes exist in pairs in the cell of the individual and the two members of a gene pair segregate (separate) from each other during gamete formation so that each gamete receives one gene of each pair. The law of segregation is applied universally . The genes are consequently not blended or mixed to each other in the hybrid, but they This ebook is exclusively for this university only. Cannot be resold/distributed. preserve their identities and transmitted unchanged in hybrids, segregate at the time of gamete formation and reappears in the offspring. This stability of genes and their transmission in unchanged condition between cells and generation is termed as purity of genes or gametes. The two alleles (dominant and recessive) of a gene pair remain together through generations without permanent influence on each other. Test of homozygous and heterozygous dominant Mendel used two types of tests to distinguish between homozygous and heterozygous dominants. These were self-fertilization and back cross (test cross). An individual heterozygous for one pair of gene ( R and r ) is called monohybrid and a cross of two individuals that are heterozygous for one pair of gene is called a monohybrid cross . This is the cross between two pure breeding individuals which differ from each other only in one character and the resulting hybrid is called monohybrid.
eBook - PDFConcise Encyclopedia of Crop Improvement
Institutions, Persons, Theories, Methods, and Histories
- Rolf Schlegel(Author)
- 2007(Publication Date)
- CRC Press(Publisher)
When the hybrids of plants differing by a single trait were self-pol-linated, three-fourths of the progeny displayed the dominating trait and one-fourth the recessive. In the next self-generation, progeny of plants displaying the recessive trait remained constant (nonsegre-gating), but those with the dominant trait produced one of two pat-terns of inheritance. One-third was constant, as in the original parent with the dominating trait, and two-thirds were segregating as in the hybrid. Thus, the 3:1 ratio in the first segregating generation (now called the F 2 generation) was broken down into a ratio of 1 (true breeding dominant) : 2 (segregating dominant as in the hybrid) : 1 (true breeding recessive). The explanation proposed was that theme 54 CONCISE ENCYCLOPEDIA OF CROP IMPROVEMENT parental plants had paired elements (e.g., AA or aa , respectively) and the hybrids of such a cross were of the constitution Aa . It took seven years to cross and score the plants to the thousand to prove the laws of inheritance. From his studies, MENDEL derived certain basic laws of heredity: 1. Hereditary factors do not combine but are passed intact. 2. Each member of the parental generation transmits only half of its hereditary factors to each offspring (with certain factors “dominant” over others). 3. Different offspring of the same parents receive different sets of hereditary factors. MENDEL, in his paper, spoke about the “law of combination of different characters” and talked about “the law of independent assort-ment.” He implied that the segregation of factors occurred in the pro-duction of sex cells. Thus, he conceptualized a genetic theory that created a new view on biology. The standard approach to unravel the mysteries of heredity was to analyze complex characters usually from wide crosses, a method that had failed for 2,000 years and was to continue to fail even when applied by the combined talents of >>> F.
eBook - PDF- Robert Jurmain, Lynn Kilgore, Wenda Trevathan, Russell Ciochon(Authors)
- 2017(Publication Date)
- Cengage Learning EMEA(Publisher)
principle of segregation Genes (alleles) occur in pairs because chromosomes occur in pairs. During gamete formation, the members of each pair of alleles separate, so that each gamete contains one member of each pair. Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-300 THE GENETIC PRINCIPLES DISCOVERED BY MENDEL 85 Today we know that meiosis explains Mendel’s principle of segregation. During meiosis, paired chromosomes, and the genes they carry, separate from each other and end up in different gametes. However, in the zygote, the full complement of chromosomes is restored, and both members of each chromosome pair are pres-ent in the offspring. Dominance and Recessiveness Mendel also realized that the “unit” for the absent characteristic (shortness) in the F 1 plants hadn’t actually disappeared. It was still there, but for some reason it wasn’t expressed. Mendel described the expression that seemed to be lost as “ recessive ,” and the expressed trait “ dominant .” Thus, the principles of dominance and reces-siveness were developed, and they remain important concepts in genetics today. As it turns out, height in garden peas is controlled by two different alleles at the same genetic locus; we’ll call it the height locus. The allele that specifies tall is dom-inant to the allele for short. (It’s worth mentioning that height isn’t controlled this way in all plants.) In Mendel’s experiments, all the parent plants had two copies of the same allele, either dominant or recessive, depending on whether they were tall or short. When two copies of the same allele are present, the individual is said to be homozygous . Thus all the tall parent plants were homozygous for the domi-nant allele and all the short parent plants were homozygous for the recessive allele. This explains why crossing tall plants with tall plants produced only tall offspring.
eBook - PDF- D. Peter Snustad, Michael J. Simmons(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
54 Chapter 3 Mendelism: The Basic Principles of Inheritance MENDELIAN SEGREGATION IN HUMAN FAMILIES In humans, the number of children produced by a couple is typically small. Today in the United States, the average is around two. In developing countries, it is six to seven. Such numbers provide nothing close to the statistical power that Mendel had in his experiments with peas. Consequently, phenotypic ratios in human families often devi- ate significantly from their Mendelian expectations. As an example, let’s consider a couple who are each heterozygous for a recessive allele that, in homozygous condition, causes cystic fibrosis, a serious disease in which breathing is impaired by an accumulation of mucus in the lungs and respiratory tract. If the couple were to have four children, would we expect exactly three to be unaffected and one to be affected by cystic fibrosis? The answer is no. Although this is a possible outcome, it is not the only one. There are, in fact, five distinct possibilities: 1. Four unaffected, none affected. 2. Three unaffected, one affected. 3. Two unaffected, two affected. 4. One unaffected, three affected. 5. None unaffected, four affected. Intuitively, the second outcome seems to be the most likely, since it conforms to Mendel’s 3:1 ratio. We can calculate the probability of this outcome, and of each of the others, by using Mendel’s principles and by treating each birth as an independent event (◾ Figure 3.14). For a particular birth, the chance that the child will be unaffected is 3/4. The probability that all four children will be unaffected is therefore (3/4) × (3/4) × (3/4) × (3/4) = (3/4) 4 = 81/256. Similarly, the chance that a particular child will be affected is 1/4; thus, the probability that all four will be affected is (1/4) 4 = 1/256. To find the probabilities for the three other outcomes, we need to recognize that each actually represents a collection of distinct events.
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 - 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 - PDF- William V Dashek, Marcia Harrison(Authors)
- 2010(Publication Date)
- CRC Press(Publisher)
MEKDEUAN GENETICS 279 TABLE 10.8 Summary of F 2 results obtained by various workers on inheritance of seed color in peas Later work on peas by other investiga- tors completely confirmed Mendel's results (Table 10. 8). What Mendel had shown and rediscoverers of Mendelism verified may be summarized thusly: 1. When parents differ in characters, the offspring resemble one parent but not the other. This is the phenomenon of dominance. 2. When the hybrid reproduces, it transmits with equal frequency either the dominant character of one parent or the recessive charac- ter of the other, but not both. This is the principle of segregation. 3. When parents differ in two or more pairs of characters, each pair shows dominance and segregation inde- pendent of the other. As a consequence all possible combina- tions of the two or more pairs can occur in the reproductive cells of the hybrid, and in their chance frequencies. This is the principle of recombination. Impact of Mendelism and its Rediscovery An important consequence of Mendel's work and its subsequent rediscovery was to replace the blending theory of inheritance with a paniculate theory of inheritance, Mendel's observation that one of the paren- tal characteristics was absent in F x hybrid and reappeared in unchanged form in the F 2 generation was inconsistent with the blend- ing theory. It was concluded from Mendel's work that traits from the parental lines were transmitted as two different elements of participate nature that retained their purity in the Fj hybrids. This gave rise to the par- ticulate theory of inheritance. CONCEPT OF DOMINANCE Earlier a gene was considered dominant if it was expressed in the heterozygous condi- tion and recessive if it was masked. Sometimes heterozygous individuals for a particular locus reveal an intermediate phe- notype. For example, bar-eyed heterozygote Drosophila individuals have a kidney- shaped eye which is different from either of the homozygous parents.
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