Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 49
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Two exchanges between two geneshave the same effect, as shown in Figure 4.13. Figure 4.13A shows a two-strand double crossover, in which thesame chromatids participate in both exchanges; no recombination of the marker genes is detectable.
When the twoexchanges have one chromatid in common (three-strand double crossover, Figure 4.13B and C), the result isindistinguishable from that of a single exchange; two products with parental combinations and two withrecombinant combinations are produced. Note that there are two types of three-strand doubles, depending on whichthree chromatids participate. The final pos-Figure 4.13Diagram showing that the result of two exchanges in the interval between two genes isindistinguishable from independent assortment of the genes, provided that the chromatidsparticipate at random in the exchanges. (A) A two-strand double crossing-over.
(B and C) The twotypes of three-strand double crossing-overs. (D) A four-strand double crossing-over.Page 141sibility is that the second exchange connects the chromatids that did not participate in the first exchange (fourstrand double crossover, Figure 4.13D), in which case all four products are recombinant.In most organisms, when double crossovers are formed, the chromatids that take part in the two exchange eventsare selected at random.
In this case, the exepected proportions of the three types of double exchanges are 1/4 fourstrand doubles, 1/2 three-strand doubles, and 1/4 two-strand doubles. This means that, on the average,(1/4)(0) + (1/2)(2) + (1/4)(4) = 2recombinant chromatids will be found among the 4 chromatids produced from meioses with two exchangesbetween a pair of genes. This is the same proportion obtained with a single exchange between the genes.
Moreover,a maximum of 50 percent recombination is obtained for any number of exchanges.In the discussion of Figure 4.13, we emphasized that, in most organisms, the chromatids taking part in doubleexchange events are selected at random. Then the maximum frequency of recombination is 50 percent. When thereis a nonrandom choice of chromatids in successive crossovers, the phenomenon is called chromatid interference.It can be seen in Figure 4.13 that, relative to a random choice of chromatids, an excess of four-strand doublecrossing-over (positive chromatid interference) results in a maximum frequency of recombination greater than 50percent; likewise, an excess of two-strand double crossing-over (negative chromatid interference) results in amaximum frequency of recombination smaller than 50 percent. Therefore, the finding that the maximum frequencyof recombination between two genes in the same chromosome is not 50 percent can be regarded as evidence forchromatid interference.
Positive chromatid interference has not yet been observed in any organism; negativechromatid interference has been reported in some fungi.Double crossing-over is detectable in recombination experiments that employ three-point crosses, which includethree pairs of alleles. If a third pair of alleles, c+ and c, is located between the two withFigure 4.14Diagram showing thattwo exchangesbetween the samechromatids and spanningthe middle pair ofalleles in a triple heterozygotewill result in a reciprocalexchange of thatpair of alleles betweenthe two chromatids.which we have been concerned (the outermost genetic markers), then double exchanges in the region can bedetected when the crossovers flank the c gene (Figure 4.14).
The two crossovers, which in this example take placebetween the same pair of chromatids, would result in a reciprocal exchange of the c+ and c alleles between thechromatids. A three-point cross is an efficient way to obtain recombination data; it is also a simple method fordetermining the order of the three genes, as we will see in the next section.4.3—Gene Mapping from Three-Point TestcrossesThe data in Table 4.1, which result from a testcross in corn with three genes in a single chromosome, illustrates theanalysis of a three-point cross. The recessive alleles of the genes in this cross are lz (for lazy or prostrate growthhabit), gl (for glossy leaf), and su (for sugary endosperm), and the multiply heterozygous parent in the cross has thegenotypeTherefore, the two classes of progeny that inherit noncrossover (parental-type) gametes are the normal plants andthose with the lazy-glossy-sugary phenotype.
These classes are far larger than any of the crossover classes. If thecombination of dominant and recessive alleles in the chromosomes of the heterozygous parent werePage 142Table 4.1 Progeny from a three-point testcross in cornPhenotype of testcrossprogenyGenotype of gametefrom hybrid parentNumberNormal (wildtype)Lz Gl Su286Lazylz Gl Su33GlossyLz gl Su59SugaryLz Gl Su4Lazy, glossylz gl Su2Lazy, sugarylz Gl Su44Glossy, sugaryLz gl Su40Lazy, glossy, sugarylz gl Su272unknown, then we could deduce from their relative frequency in the progeny that the noncrossover gametes wereLz Gl Su and lz gl su.
This is a point important enough to state as a general principle:In any genetic cross involving linked genes, no matter how complex, the two most frequent types ofgametes with respect to any pair of genes are nonrecombinant; these provide the linkage phase (cis versustrans) of the alleles of the genes in the multiply heterozygous parent.In mapping experiments, the gene sequence is usually not known. In this example, the order in which the threegenes are shown is entirely arbitrary.
However, there is an easy way to determine the correct order from three-pointdata. Simply identify the genotypes of the double-crossover gametes produced by the heterozygous parent andcompare them with the nonrecombinant gametes. Because the probability of two simultaneous exchanges isconsiderably smaller than of either single exchange, the double-crossover gametes will be the least frequent types.Table 4.1 shows that the classes composed of four plants with the sugary phenotype and two plants with the lazyglossy phenotype (products of the Lz Gl su and Lz gl Su gametes, respectively) are the least frequent and thereforeconstitute the double-crossover progeny.The effect of double crossing-over, as Figure 4.14 shows, is to interchange the members of the middle pairof alleles between the chromosomes.This means that if the parental chromosomes areand the double-crossover chromosomes arethen Su and su are interchanged by the double crossing-over and must be the middle pair of alleles.
Therefore, thegenotype of the heterozygous parent in the cross should be written aswhich is now diagrammed correctly with respect to both the order of the genes and the array of alleles in thehomologous chromosomes. A two-strand double crossover between chromatids of these parental types isdiagrammed below, and the products can be seen to correspond to the two types of gametes identified in the data asthe double crossovers.From this diagram, it can also be seen that the reciprocal products of a single crossover between lz and su would beLz su gl and Lz Su Gl and that the products of a single exchange between su and gl would be Lz Su gl and lz su Gl.We can now summarize the data in a more informative way, writing the genes in correct order and identifying thenumbers of the different chromosome types produced by the heterozygous parent that are present in the progeny.Note that each class of single recombinants consists of two reciprocal productsPage 143and that these are found in approximately equal frequencies (40 versus 33 and 59 versus 44).
This observationillustrates an important principle:The two reciprocal products that result from any crossover, or any combination of crossovers, are expectedto appear in approximately equal frequencies among the progeny.In calculating the frequency of recombination from the data, remember that the double-recombinant chromosomesresult from two exchanges, one in each of the chromosome regions defined by the three genes. Therefore,chromosomes that are recombinant between lz and su are represented by the following chromosome types:Lzsugl40lzSuGl33LzsuGl4lzSugl279That is, 79/740, or 10.7 percent, of the chromosomes recovered in the progeny are recombinant between the lz andsu genes, so the map distance between these genes is 10.7 map units or 10.7 centimorgans. Similarly, thechromosomes that are recombinant between su and gl are represented byLzSugl59lzsuGl44LzsuGl4lzSugl2109The recombination frequency between this second pair of genes is 109/740, or 14.8 percent, so the map distancebetween them indicated by these data is 14.8 map units or 14.8 centimorgans.
The genetic map of the chromosomesegment in which the three genes are located is thereforeThe error that students most commonly make as they are learning how to interpret three-point crosses is to forget toinclude the double recombinants when calculating the recombination frequency between adjacent genes. You cankeep from falling into this trap by remembering that the double recombinant chromosomes have singlerecombination in both regions.Chromosome Interference in Double Crossing-overThe detection of double crossing-over makes it possible to determine whether exchanges in two different regions ofa pair of chromosomes are formed independently of each other. Using the information from the example with corn,we know from the recombination frequencies that the probability of recombination is 0.107 between lz and su and0.148 between su and gl.
If crossing-over is independent in the two regions (which means that the formation of oneexchange does not alter the probability of the second exchange), then the probability of an exchange in bothregions is the product of these separate probabilities, or 0.107 × 0.148 = 0.0158 (1.58 percent). This implies that ina sample of 740 gametes, the expected number of double crossovers would be 740 × 0.0158, or 12, whereas thenumber actually observed was only 6. Such deficiencies in the observed number of double crossovers are commonand identify a phenomenon called chromosome interference, in which crossing-over in one region of achromosome reduces the probability of a second crossover in a nearby region. Because chromosome interference isnearly universal, and chromatid interference is virtually unknown, the term interference, when used withoutqualification, almost always refers to chromosome interference.The coefficient of coincidence is the observed number of double recombinant chromosomes divided by theexpected number.
Its value provides a quantitative measure of the degree of interference, defined asFrom the data in our example, the coefficient of coincidence is 6/12 = 0.50, whichPage 144means that the observed number of double crossovers was only 50 percent of the number we would expect toobserve if crossing-over in the two regions were independent. The value of the interference depends on the distancebetween the genetic markers and on the species. In some species, the interference increases as the distance betweenthe two outside markers becomes smaller, until a point is reached at which double crossing-over is eliminated; thatis, no double crossovers are found, and the coefficient of coincidence equals 0 (or, to say the same thing, theinterference equals 1). In Drosophila, this distance is about 10 map units.
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