Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 48
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The eggs contain either the attached-X or the Ychromosome, which combine at random with X-bearing or Y-bearing sperm.Genotypes with either three X chromosomes or no X chromosomesare lethal. Note that a male fly receives its X chromosome from its father and itsY chromosome from its mother—the opposite of the usual situation inDrosophila.Figure 4.11Diagram showing that crossing-over must take place at the four-strand stage in meiosis toproduce a homozygous attached-X chromosome from one that is heterozygousfor an allele. To yield homozygosity, the exchange must take place between the centromere and the gene.Page 137Connection Dos XXLilian V.
Morgan 1922Columbia University,New York, New YorkNon-crisscross Inheritance in Drosophila melanogasterLilian V. Morgan was a first-rate geneticist long associated with T. H. Morgan as his collaborator and wife. Shediscovered the first attached-X chromosome as a single exceptional female in a routine mapping cross. Sherealized instinctively that this female was extremely important. There is an old story, of uncertain validity, that thefemale temporarily escaped, causing consternation and a mad search by everyone in the laboratory, until finally itwas found resting on a window pane.
The attached-X chromosome is still one of the most important genetic toolsavailable to Drosophila geneticists.A complete reversal of the ordinary crisscross inheritance of recessive X-linked characters occurs in a line ofDrosophila recently obtained. In ordinary X-linked inheritance, the recessive X-linked characters of the mother aretransmitted to the sons, while the daughters show the dominant allele of the father. In the present case, thedaughters show a recessive X-linked character of the mother and the sons show the dominant allele of the father.The reversal is explicable on the assumption that the two X-chromosomes of the mother are united and behave inmeiosis as a single body.
The cytological evidence verifies the genetic deduction. The eggs of theseThe reversal is explicable on the assumption that the two X-chromosomes of the mother areunited and behave in meiosis as a single body.females do have two united X-chromosomes. . . . [A single female fly with a yellow abdomen was found in a crossbetween a homozygous nonyellow (gray) female and a yellow male.] She was mated to a gray male and produced43 daughters and 59 sons. The daughters were, without exception, all yellow and the sons were all gray. Theconclusion was at once evident that the [mother] had received from its father two yellow-bearing chromosomes,inseparable from one another, and that these inseparable chromosomes were transmitted together to the nextgeneration producing (wherever they occurred) females, because there were always two of them.
No male offspringcould be yellow, because no single yellow chromosome was transmitted. . . . The F1 females were fertile. . . . Thedaughters were all yellow, but differed from their yellow mother in having, besides the ''yellow-bearing" doublechromosome [the attached-X], a Y-bearing chromosome from their father. . . . The genetic behavior of the line offlies having the two inseparable X chromosomes is in entire accord with the condition of the chromosomes as seenin cytological preparations.
. . . The origin of the [attached-X] can be explained if at some division inspermatogenesis of the father (perhaps at the equational division) the two halves of the X chromosome failed tobecome completely detached, but remained fastened together at one of their ends, producing the V-shapedchromosome found in the germ cells of the female descendants.Source: Biological Bulletin 42: 267-274crossing-over between the X-chromosome arms can yield attached-X products in which the recessive allele is presenin both arms of the attached-X chromosome (Figure 4.11). Hence, attached-X females that are heterozygous canproduce some female progeny that are homozygous for the recessive allele.
The frequency with which homozygosityis observed increases with increasing map distance of the gene from the centromere. From the diagrams in Figure4.11, it is clear that homozygosity can result only if the crossover between the gene and the centromere takes placeafter the chromosome has duplicated. The implication of finding homozygous attached-X female progeny is thereforthat crossing-over takes place at the four-strand stage of meiosis. If this were not the case, and crossing-overhappened before duplication of the chromosome (at the two-strand stage), it would result only in a swap of the allelebetween the chromosome arms and would never yield the homozygous products that are actually observed.The Molecular Basis of Crossing-overAs we will see in Chapter 6, each chromosome in a eukaryote contains a single, long molecule of duplex DNAcomplexed with proteins that undergoes a process of condensation, forming a hierarchy of coils upon coils thatbecomes progressively tighter as the chromosome progresses through nuclear division and reaches a state ofmaximum condensation at metaphase.
Crossing-over along a chromosome must therefore correspond to somePage 138type of exchange of genetic information between DNA molecules.The first widely accepted model of recombination between DNA molecules was proposed by Robin Holliday in1964. Although it is overly simplistic in some of the details, the model has formed the basis of more realisticmodels favored today that account for most observations related to recombination.
These models, and the evidenceon which they are based, are discussed in detail in Chapter 13 in the context of DNA breakage and repair. It is,however, appropriate to introduce the Holliday model at this point to connect crossing-over observed inchromosomes to exchange between DNA molecules as envisaged in the Holliday model.An outline of the Holliday model is illustrated in Figure 4.12. The DNA molecules depicted are those present in thechromatids that participate in the recombination event. The DNA duplexes in the other two chromatids, which arealso present at the time of recombination, are not shown.
The exchange is initiated by a single-stranded break ineach molecule (Figure 4.12A), the ends of which are joined crosswise (Figure 4.12B). DNA is a dynamic moleculethat "breathes" in the sense that local regions of paired bases frequently come apart and form again. Such"breathing" in the region of the exchange allows the molecules to exchange pairing partners along a region near thepoint of exchange (Figure 4.12C); the exchange of pairing partners is called branch migration. At any time, breaksat the positions of the arrows in part C, followed by crosswise rejoining, result in separate DNA molecules (Figure4.12D) that are recombinant for the outside genetic markers—namely, Ab and aB.
In part E, the second pair ofbreaks rejoin to resolve the interconnected Holliday structure in part C.We need to make one additional comment relative to scale. The molecular events in Figure 4.12 aresubmicroscopic, and the Holliday structure can be observed only under favorable conditions through an electronmicroscope. Therefore, the cross-shaped exchange between the DNA strands indicated in part C is invisiblethrough the light microscope. What, then, is a chiasma, the cross-shaped structure that connects nonsisterchromatids in a bivalent? In pachytene, at the time of crossing-over, the chromatids are already condensed enoughto be visible through the light microscope.
In Figure 4.12, the DNA is shown in an elongated form rather than inthe highly convoluted form actually present in condensed chromatin. The events in Figure 4.12 take place in a localregion of DNA where the molecules are able to undergo the exchange. The events themselves are invisible.However, the resulting connection between the chromatids forms a visible chiasma between nonsister chromatids.Like a loose knot sliding along a rope, a chiasma can also slide along a chromosome, so the physical position of achiasma may not necessarily represent the physical location of the DNA exchange that led to its formation.Multiple Crossing-overWhen two genes are located far apart along a chromosome, more than one crossover can be formed between themin a single meiosis, and this complicates the interpretation of recombination data.
The probability of multiplecrossovers increases with the distance between the genes. Multiple crossing-over complicates genetic mappingbecause map distance is based on the number of physical exchanges that are formed, and some of the multipleexchanges between two genes do not result in recombination of the genes and hence are not detected. As we saw inFigure 4.6, the effect of one crossover can be canceled by another crossover farther along the way. If twoexchanges between the same two chromatids take place between the genes A and B, then their net effect will be thatall chromosomes are nonrecombinant, either AB or ab. Two of the products of this meiosis have an interchange oftheir middle segments, but the chromosomes are not recombinant for the genetic markers, and so are geneticallyindistinguishable from noncrossover chromosomes. The possibility of such canceling events means that theobserved recombination value is an underestimate of the true exchange frequency and the map distance betweenthe genes.
In higher organisms, double crossing-over is effectively precluded in chromosome segments that aresufficiently short. Therefore,Page 139Figure 4.12The Holliday model of recombination. (A) In the participating DNAmolecules, the exchange process is initiated by a single-stranded breakin one strand of each duplex. (B) The ends of the broken strands arejoined crosswise, resulting in a connection between the molecules.(C) The newly joined strands "unzip" a little and exchange pairingpartners (the exchange of pairing partners is called branch migration).The exchange can be resolved by the breaking and rejoining of theouter strands.
(D) In resolving the structure, the nicked outer strandsexchange places. (E) Sealing of the gaps results in molecules thatare recombinant for the outside genetic markers (A band a B).Page 140by using recombination data for closely linked genes to build up genetic linkage maps, we can avoid multiplecrossovers that cancel each other's effects.The minimum recombination frequency between two genes is 0. The recombination frequency also has amaximum:No matter how far apart two genes may be, the maximum frequency of recombination between any twogenes is 50 percent.Fifty percent recombination is the same value that would be observed if the genes were on nonhomologouschromosomes and assorted independently. The maximum frequency of recombination is observed when the genesare so far apart in the chromosome that at least one crossover is almost always formed between them.Figure 4.3B, shows that a single exchange in every meiosis would result in half of the products having parentalcombinations and the other half having recombinant combinations of the genes.
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