Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 46
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Each crossing-over ismanifested physically as a chiasma, or cross-shaped configuration, between homologous chromosomes; chiasmataare observed in prophase I of meiosis (Chapter 3). Each chiasma results from the breaking and rejoining ofchromatids during synapsis, with the result that there is an exchange of corresponding segments between them. Thetheory of crossing-over is that each chiasma results in a new association of genetic markers. This process isillustrated in Figure 4.3. When there is no crossing-over (Figure 4.3A), the alleles present in each homologouschromosome remain in the same combination. When crossing-over does take place (Figure 4.3B), the outermostalleles in two of the chromatids are interchanged (recombined).The unit of distance in a genetic map is called a map unit; 1 map unit is equal to 1 percent recombination. Forexample, two genes that recombine with a frequency of 3.5 percent are said to be located 3.5 map units apart.
Onemap unit is also called a centimorgan, abbreviated cM, in honor of T. H. Morgan. A distance of 3.5 map unitstherefore equals 3.5 centimorgans and indicates 3.5 percent recombination between the genes. For ease ofreference, we list the four completely equivalent ways in which a genetic distance between two genes may berepresented.• As a frequency of recombination (in the foregoing example, 0.035)• As a percent recombination (here 3.5 percent)• As a map distance in map units (in this case, 3.5 map units)• As a map distance in centimorgans (here 3.5 centimorgans, abbreviated 3.5 cM)Physically, 1 map unit corresponds to a length of the chromosome in which, on theFigure 4.3Diagram illustrating crossing-over between two genes.
(A) When there is no crossing-over betweentwo genes, the alleles are not recombined. (B) When there is crossing-over between them, theresult of the crossover is two recombinant and two nonrecombinant products, because the exchangeis between only two of the four chromatids.Page 129Figure 4.4Diagram of chromosomal configurations in 50 meiotic cells, in which one has a crossover betweentwo genes. (A) The 49 cells without a crossover result in 98 A B and 98 a b chromosomes; theseare all nonrecombinant.
(B) The cell with a crossover yields chromosomes thatare A B, A b, a B, and a b, of which the middle two types are recombinantchromosomes. (C) The recombination frequency equals 2/200, or 1 percent, which is alsocalled 1 map unit or 1 cM. Hence 1 percent recombination means that 1 meiotic cell in 50 has acrossover in the region between the genes.average, one crossover is formed in every 50 cells undergoing meiosis. This principle is illustrated in Figure 4.4. Ifone meiotic cell in 50 has a crossing-over, the frequency of crossing-over equals 1/50, or 2 percent.
Yet thefrequency of recombination between the genes is 1 percent. The correspondence of 1 percent recombination with 2percent crossing-over is a little confusing until you consider that a crossover results in two recombinant chromatidsand two nonrecombinant chromatids (Figure 4.4). A frequency of crossing-over of 2 percent means that of the 200chromosomes that result from meiosis in 50 cells, exactly 2 chromosomes (the two involved in the exchange) arerecombinant for genetic markers spanning the particular chromosome segment. To put the matter in another way, 2percent crossing-over corresponds to 1 percent recombination because only half the chromatids in each cell with anexchange are actually recombinant.In situations in which there are genetic markers along the chromosome, such as the A, a and B, b pairs of alleles inFigure 4.4, recombination between the marker genes takes place only when crossing-over occurs between thegenes.
Figure 4.5 illustrates a case in which crossing-over takes place between the gene A and the centromere,rather than between the genes A and B. The crossing-over does result in the physical exchange of segmentsbetween the innermost chromatids. However, because it is located outside the region between A and B, all of theresulting gametes must carry either the A B or a b allele combinations. These are nonrecombinant chromosomes.The presence of the crossing-over is undetected because it is not in the region between the genetic markers.In some cases, the region between genetic markers is large enough that two (or even more) crossovers can beformed in a single meiotic cell. One possible configuration for two crossovers is shown in Figure 4.6.
In thisexample, both crossovers are between the same pair of chromatids. The result is that there is a physical exchange ofa segment of chromosome between the marker genes, but the double crossover remains undetected because themarkers themselves are not recombined.
The absence of recombination results from the fact that the secondcrossover reverses thePage 130Figure 4.5Crossing-over outside the region between two genes is not detectable through recombination.Although a segment of chromosome is exchanged, the genetic markers stay in thenonrecombinant configurations, in this case A B and a b.effect of the first, insofar as recombination between A and B is concerned. The resulting chromosomes are either AB or a b,] both of which are nonrecombinant.Given that double crossing-over in a region between two genes can remain undetected because it does not result inrecombinant chromosomes, there is an important distinction between the distance between two genes as measuredby the recombination frequency and as measured in map units. Map units measure how much crossing-over takesplace between the genes.
For any two genes, the map distance between them equals one-half times the averagenumber of crossover events that take place in the region per meiotic cell. The recombination frequency, on theother hand, reflects how much recombination is actually observed in a particular experiment. Double crossoversthat do not yield recombinant gametes, such as the one in Figure 4.6, do contribute to the map distance but do notcontribute to the recombination frequency.
The distinction is important only when the region in question is largeenough that double crossingover can occur. If the region between the genes is so short that no more than onecrossover can be formed in the region in any one meiosis, then map units and recombination frequencies are thesame (because there are no multiple crossovers that can undo each other). This is the basis for defining a map unitas being equal to 1 percent recombination.
Over an interval so short as to yield 1 percent observed recombination,multiple crossovers are usually precluded, so the map distance equals the recombination frequency in this case.Figure 4.6If two crossovers take place between marker genes, and both involve the same pair of chromatids,then neither crossover is detected because all of the resulting chromosomes are nonrecombinant A B or a b.Page 131Connection Genes All in a RowAlfred H. Sturtevant 1913Columbia University,New York, New YorkThe Linear Arrangement of Six Sex-Linked Factors in Drosophila, As Shown by Their Mode of AssociationGenetic mapping remains the cornerstone of genetic analysis.
It is the principal technique used in modern humangenetics to identify the chromosomal location of mutant genes associated with inherited diseases, as we saw withHuntington disease in Chapter 2. The genetic markers used in human genetics are homologous DNA fragmentsthat differ in length from one person to the next, but the basic principles of genetic mapping are the same as thoseoriginally enunciated by Sturtevant. In this excerpt, we have substituted the symbols presently in use for thegenes, y (yellow body), w (white eyes), v (vermilion eyes), m (miniature wings), and r (rudimentary wings).
(Thesixth gene mentioned is another mutant allele of white, now called white-eosin.) In this paper, Sturtevant uses theterm crossing-over instead of recombination and crossovers instead of recombinant chromosomes. We haveretained his original terms but, in a few cases, have put the modern equivalent in brackets.Morgan, by crossing white eyed, long winged flies to those with red eyes and rudimentary wings (the new sexlinked character), obtained, in F2, white eyed rudimentary winged flies. This could happen only if "crossingover" [recombination] is possible; which means, on the assumption that both of these factors are in the Xchromosome, that an interchange of materials between homologous chromosomes occurs (in the female only,since the male has only one X chromosome).
A point not noticed at this time came out later in connection withother sex-linked factors in Drosophila. It became evident that some of the sex-linked factors are associated, i. e.,that crossing-over does not occur freely between some factors, as shown by the fact that the combinations presentin the F1 flies are much more frequent in the F2 thanThese results form a new argument in favor of the chromosome view of inheritance, since theystrongly indicate that the factors investigated are arranged in a linear series.are new combinations of the same characters. This means, on the chromosome view, that the chromosomes, or atleast certain segments of them, are much more likely to remain intact during meiosis than they are to interchangematerials.
. . . It would seem, if this hypothesis be correct, that the proportion of "crossovers" [recombinantchromosomes] could be used as an index of the distance between any two factors. Then by determining thedistances (in the above sense) between A and B and between B and C, one should be able to predict AC. .
. . Justhow far our theory stands the test is shown by the data below, giving observed percent of crossovers [recombinantchromosomes] and the distances calculated [from the summation of shorter intervals].Factorsy-vy-my-rw-mw-rCalculatedObservedpercentage ofdistance30.733.757.632.756.6crossovers32.235.237.633.745.2It will be noticed at once that the longer distances, y—r and w—r, give smaller per cent of crossovers, than thecalculation calls for. This is a point which was to be expected and is probably due to the occurrence of two breaksin the same chromosome, or "double crossing-over." But in the case of the shorter distances the correspondencewith expectation is perhaps as close as was to be expected with the small numbers that are available. .
. . It hasbeen found possible to arrange six sex-linked factors in Drosophila in a linear series, using the number ofcrossovers per 100 cases [the frequency of recombination] as an index of the distance between any two factors. Asource of error in predicting the strength of association between untried factors is found in double crossing-over.The occurrence of this phenomenon is demonstrated.
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