Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 20
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It is shown that this series is described by the expression: A + 2Aa + a, in which A and a signify theforms with constant differing traits, and Aa the form hybrid for both. The series contains four individuals in threedifferent terms. In their production, pollen and germinal cells of form A and a participate, on the average, equally infertilization; therefore each form manifests itself twice, since four individuals are produced.Participating in fertilization are thus:Pollen cells A + A + a + aGerminal cells A + A + a + aWhether the plan by which the individual experiments were set up and carried out was adequate to theassigned task should be decided by a benevolent judgmentIt is entirely a matter of chance which of the two kinds of pollen combines with each single germinal cell.
Howeveraccording to the laws of probability, in an average of many cases it will always happen that every pollen form A anda will unite equally often with every germinal-cell form A and a; therefore, in fertilization, one of the two pollencells A will meet a germinal cell A, the other a germinal cell a, and equally, one pollen cell a will become associatewith a germinal cell A, and the other a.The result of fertilization can be visualized by writing the designations for associated germinal and pollen cells in thform of fractions, pollen cells above the line, germinal cells below. In the case under discussion one obtainsIn the first and fourth terms germinal and pollen cells are alike; therefore the products of their association must beconstant, namely A and a; in the second and third, however, a union of the two differing parental traits takes placeagain, therefore the forms arising from such fertilizations are absolutely identical with the hybrid from which theyderive. Thus, repeated hybridization takes place.
The striking phenomenon, that hybrids are able to produce, inaddition to the two parental types, progeny that resemble themselves is thus explained: Aa and aA both give the samassociation, Aa, since, as mentioned earlier, it makes no difference to the consequence of fertilization which of thetwo traits belongs to the pollen and which to the germinal cell. ThereforeThis represents the average course of self-fertilization of hybrids when two differing traits are associated in them. Iindividual flowers and individual plants, however, the ratio in which the members of the series are formed may besubject to not insignificant deviations.
. . . Thus it was proven experimentally that, in Pisum, hybrids form differentkinds of germinal and pollen cells and that this is the reason for the variability of their offspring.Source: Verhandlungen des naturforschenden den Vereines in Brünn 4: 3–47Page 39deductions: Two plants with the same outward appearance, such as round seeds, might nevertheless differ in theirhereditary makeup as revealed by the types of progeny observed when they are crossed. For example, in the truebreeding round variety, the genetic composition of the seeds is AA, whereas in the F1 hybrid seeds of the round ×wrinkled cross, the genetic composition of the seeds is Aa.But how could the genetic hypothesis be tested? Mendel realized that a key prediction of his hypothesis concernedthe genetic composition of the round seeds in the F2 generation. If the hypothesis is correct, then one-third of theround seeds should have the genetic composition AA and two-thirds of the round seeds should have the geneticcomposition Aa.
This principle is shown in Figure 2.5. The ratio of AA: Aa: aa in the F2 generation is 1 : 2 : 1, butif we disregard the recessives, then the ratio of AA: Aa is 1 : 2; in other words, 1/3 of the round seeds are AA and2/3 are Aa. Upon self-fertilization, plants grown from the AA types should be true breeding for round seeds,whereas those from the Aa types should yield round and wrinkled seeds in the ratio 3 : 1. Furthermore, among thewrinkled seeds in the F2 generation, all should have the genetic composition aa, and so, upon self-fertilization, theyshould be true breeding for wrinkled seeds.For several of his traits, Mendel carried out self-fertilization of the F2 plants in order to test these predictions.
Hisresults for round versus wrinkled seeds are summarized in the diagram below:Figure 2.5In the F2 generation, the ratio of AA : Aa is 1 : 2.Therefore, among those seeds that are round, 1/3should be AA and 2/3 should be Aa.As predicted from Mendel's genetic hypothesis, the plants grown from F2 wrinkled seeds were true breeding forwrinkled seeds. They produced only wrinkled seeds in the F3 generation.
Moreover, among 565 plants grown fromF2 round seeds, 193 were true breeding, producing only round seeds in the F3 generation, whereas the other 372plants produced both round and wrinkled seeds in a proportion very close to 3 : 1. The ratio 193 : 372 equals 1 :1.93, which is very close to the ratio 1 : 2 of AA : Aa types predicted theoretically from the genetic hypothesis inFigure 2.4. Overall, taking all of the F2 plants into account, the ratio of genetic types observed was very close to thepredicted 1 : 2 : 1 of AA : Aa : aa expected from Figure 2.4.Page 40The Principle of SegregationThe diagram in Figure 2.4 is the heart of Mendelian genetics. You should master it and be able to use it to deducethe progeny types produced in crosses. Be sure you thoroughly understand the meaning, and the biological basis, ofthe ratios 3 : 1 and 1 : 2 and 1: 2 : 1.
The following list highlights Mendel's key assumptions in formulating hismodel of inheritance.1. For each of the traits that Mendel studied, a pea plant contains two hereditary determinants.2. For each pair of hereditary determinants present in a plant, the members may be identical (for example, AA) ordifferent (for example, Aa).3. Each reproductive cell (gamete) produced by a plant contains only one of each pair of hereditary determinants(that is, either A or a).4. In the formation of gametes, any particular gamete is equally likely to include either hereditary determinant(hence, from an Aa plant, half the gametes contain A and the other half contain a).5.
The union of male and female reproductive cells is a random process that reunites the hereditary determinants inpairs.The essential feature of Mendelian genetics is the separation, technically called segregation, in unaltered form, ofthe two hereditary determinants in a hybrid plant in the formation of its reproductive cells (points 3 and 4 in theforegoing list). The principle of segregation is sometimes called Mendel's first law, although Mendel never usedthis term.The Principle of Segregation: In the formation of gametes, the paired hereditary determinants separate(segregate) in such a way that each gamete is equally likely to contain either member of the pair.Apart from the principle of segregation, the other key assumption, implicit in points 1 and 5 in the list, is that thehereditary determinants are present as pairs in both the parental organisms and the progeny organisms but as singlecopies in the reproductive cells.Important Genetic TerminologyOne of the handicaps under which Mendel wrote was the absence of an established vocabulary of terms suitable fordescribing his concepts.
Hence he made a number of seemingly elementary mistakes, such as occasionallyconfusing the outward appearance of an organism with its hereditary constitution. The necessary vocabulary wasdeveloped only after Mendel's work was rediscovered, and it includes the following essential terms.1. A hereditary determinant of a trait is called a gene.2. The different forms of a particular gene are called alleles. In Figure 2.4, the alleles of the gene for seed shape areA for round seeds and a for wrinkled seeds. A and a are alleles because they are alternative forms of the gene forseed shape. Alternative alleles are typically represented by the same letter or combination of letters, distinguishedeither by uppercase and lowercase or by means of superscripts and subscripts or some other typographic identifier.3.
The genotype is the genetic constitution of an organism or cell. With respect to seed shape in peas, AA, Aa, andaa are examples of the possible genotypes for the A and a alleles. Because gametes contain only one allele of eachgene, A and a are examples of genotypes of gametes.4. A genotype in which the members of a pair of alleles are different, as in the Aa hybrids in Figure 2.4, is said tobe heterozygous; a genotype in which the two alleles are alike is said to be homozygous. A homozygous organismmay be homozygous dominant (AA) or homozygous recessive (aa). The terms homozygous and heterozygous can-Page 41not apply to gametes, which contain only one allele of each gene.5. The observable properties of an organism constitute its phenotype. Round seeds, wrinkled seeds, yellow seeds,and green seeds are all phenotypes.
The phenotype of an organism does not necessarily tell you anything about itsgenotype. For example, a seed with the phenotype "round" could have the genotype AA or Aa.Verification of Mendelian Segregation by the TestcrossA second way in which Mendel tested the genetic hypothesis in Figure 2.4 was by crossing the F1 heterozygousgenotypes with plants that were homozygous recessive. Such a cross, between an organism that is heterozygous forone or more genes (for example, Aa), and an organism that is homozygous for the recessive alleles (for example,aa), is called a testcross. The result of such a testcross is shown in Figure 2.6.
Because the heterozygous parent isexpected to produce A and a gametes in equal numbers, whereas the homozygous recessive produces only agametes, the expected progeny are 1/2 with the genotype Aa and 1/2 with the genotype aa. The former have thedominant phenotype (because A is dominant to a) and the latter have the recessive phenotype. A testcross is oftenextremely useful in genetic analysis becauseIn a testcross, the relative frequencies of the different gametes produced by the heterozygous parent can beobserved directly in the phenotypes of the progeny, because the recessive parent contributes only recessivealleles.Mendel carried out a series of testcrosses with the genes for round versus wrinkled seeds, yellow versus greenseeds, purple versus white flowers, and long versus short stems.
The results are shown Table 2.2. In all cases, theratio of phenotypes among the progeny is very close to the 1 : 1 ratio expected from segregation of the alleles in theheterozygous parent.Figure 2.6In a testcross of an Aa heterozygous parent with an aa homozygousrecessive, the progeny are Aa and aa in the ratio of 1 : 1. Atestcross shows the result of segregation.Another valuable type of cross is a backcross, in which hybrid organisms are crossed with one of the parentalgenotypes. Backcrosses are commonly used by geneticists and by plant and animal breeders, as we will see in laterchapters. Note that the testcrosses in Table 2.2 are also backcrosses, because in each case, the F1 heterozygousparent came from a cross between the homozygous dominant and the homozygous recessive.Table 2.2 Mendel's testcross resultsTestcross (F1 heterozygote ×homozygous recessive)Round × wrinkled seedsYellow × green seedsProgenyfrom testcross193 round,192 wrinkled196 yellow,Ratio1.01 : 1Purple × white flowersLong × short stems189 green1.04 : 185 purple,81 white1.05 : 187 long79 short1.10 : 1Page 422.2—Segregation of Two or More GenesMendel also carried out experiments in which he examined the inheritance of two or more traits simultaneously todetermine whether the same pattern of inheritance applied to each pair of alleles separately when more than oneallelic pair was segregating in the hybrids.
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