Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 53
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Thesefour arrangements, shown in Figure 4.27B, are A a A a, a A a A, A a a A, and a A A a.The percentage of asci with second-division segregation patterns for a gene can be used to map the gene withrespect to its centromere. For example, let us assume that 30 percent of a sample of asci from a cross have asecond-division segregation pattern for the A and a alleles.
This means that 30 percent of the cells undergoingPage 157Figure 4.27First-and second-division segregation in Neurospora. (A) First-division segregation patterns are foundin the ascus when crossing-over between the gene and centromere does not take place. The allelesseparate (segregate) in meiosis I. Two spore patterns are possible, depending on the orientation of thepair of chromosomes on the first-division spindle.
(B) Second-division segregation patterns are foundin the ascus when crossing-over between the gene and the centromere delays separation of A froma until meiosis II. Four patterns of spores are possible, depending on the orientationof the pair of chromosomes on the first-division spindle and that of the chromatids of each chromosomeon the second-division spindle.meiosis had a crossover between the A gene and its centromere. Furthermore, in each cell in which crossing-overtakes place, two of the chromatids are recombinant and two are nonrecombinant.
In other words, a frequency ofcrossing-over of 30 percent corresponds to a recombination frequency of 15 percent. By convention, map distancerefers to the frequency of recombinant meiotic products rather than to the frequency of cells with crossovers.Therefore, the map distancePage 158between a gene and its centromere is given by the equationThis equation is valid as long as the gene is close enough to the centromere that multiple crossing-over can beneglected.
Reliable linkage values are best determined for genes that are near the centromere. The location of moredistant genes is then accomplished by the mapping of these genes relative to genes nearer the centromere.If a gene is far from its centromere, crossing-over between the gene and its centromere will be so frequent that theA and a alleles become randomized with respect to the four chromatids. The result is that the six possible sporearrangements shown in Figure 4.27 are all equally frequent.
This equal likelihood reflects the patterns that canresult from choosing randomly among 2 A and 2 a spore pairs in the ascus, as shown by the branching diagram inFigure 4.28. Therefore, in the absence of chromatid interference,The maximum frequency of second-division segregation asci is 2/3.4.6—Mitotic RecombinationGenetic exchange can also take place in mitosis, although at a frequency about 1000-fold lower than in meiosis andprobably by a somewhat different mechanism than meiotic recombination.
The first evidence for mitoticrecombination was obtained by Curt Stern in 1936 in experiments with Drosophila, but the phenomenon has beenstudied most carefully in fungi such as yeast and Aspergillus, in which the frequencies are higher than in mostother organisms. Genetic maps can be constructed from mitotic recombination frequencies.
In some organisms inwhich a sexual cycle is unknown, mitotic recombination is the only method of obtaining linkage data. In organismsin which both meiotic recombination and mitotic recom-Figure 4.28Diagram showing the result of free recombination between an allelic pair, A and a, and the centromere. Thefrequency of second-division segregation equals 2. This is the maximum frequency of second-divisionsegregation, provided that there is no chromatid interference.Page 159bination are found, mitotic recombination is always at a much lower frequency. In mitotic recombination, therelative map distances between particular genes sometimes correspond to those based on meiotic recombinationfrequencies, but for unknown reasons, the distances are often markedly different.
The discrepancies may reflectdifferent mechanisms of exchange and perhaps a nonrandom distribution of potential sites of exchange.The genetic implications of mitotic recombination are illustrated in Figure 4.29. Each of the homologouschromosomes has two genetic markers distal to the site of the breakage and reunion. At the following anaphase,each centromere splits and the daughter cells receive one centromere of each color along with the chromatidattached to it. The separation can happen in two ways, as shown on the right. If the two nonexchanged and the twoexchanged chromatids go together (top), the result of the exchange is not detectable genetically.
However, if eachnonexchanged chromatid goes with one of the exchanged chromatids, then the result is that the region of thechromosome distal to the site of the exchange becomes homozygous. With an appropriate configuration of geneticmarkers, the exchange is detectable as a twin spot in which cells of one homozygous genotype (in this example, aBaB) are adjacent to cells of a different homozygous genotype (in this example, Ab Ab).
In the original experimentswith Drosophila, Stern used the X-linked markers A = y+ and a = y (y is yellow body color) along with B = sn+ andb = sn (sn is singed bristles). In females, the result of mitotic recombination was observed as a twin spot in which apatch of cuticle with a yellow color and normal bristles was adjacent to a patch of cuticle with normal color andsinged bristles.The rate of mitotic recombination can be increased substantially through x-ray treatment and the use of certainmutations.Figure 4.29Mitotic recombination. Homologous chromosomes in prophase of mitosis are shown in light anddark blue.
At anaphase, each centromere splits, and each daughter cell receives one centromere ofeach color and its attached chromatid. When a rare reciprocal exchange takes place between nonsister chromatids of homologous chromosomes, the daughter cells can be either of two types. If thenonexchanged chromatids and the exchanged chromatids go together (top right), both cells aregenetically like the parent; in this example, A b/a B. If the nonexchanged and exchanged chromatids go together (bottom right), the result is that alleles distal to the point of exchange becomehomozygous. In this example, one daughter cell is a B/a B and the other A b/A b.
Adjacent cell lineages are therefore homozygous for either aa or bb and can be detected phenotypically as a ''twinspot" of aa somatic tissue adjacent to bb somatic tissue.Page 160Mitotic recombination is a useful tool in genetics because it results in the production of somatic mosaics,organisms that contain two or more genetically different types of tissue. For example, the mosaic Drosophilafemales discussed in the previous paragraph consist of predominantly wildtype tissue but include some patches ofy+ sn/y+ sn and some patches of y sn+/y sn+.
Each twin spot of mutant tissue derives from repeated division of asingle cell that became homozygous through a mitotic exchange. Hence the mutant tissue resulting from mitoticrecombination can be used to trace the movement and differentiation of particular cell lineages in development.4.7—Recombination Within GenesGenetic analyses and microscopic observations made before 1940 led to the view that a chromosome was a lineararray of particulate units (the genes) joined in some way resembling a string of beads.
The gene was believed to bethe smallest unit of genetic material capable of alteration by mutation and the smallest unit of inheritance.Recombination had been seen in all regions of the chromosome but not within genes, and the idea of theindivisibility of the gene developed. This classical concept of a gene began to change when geneticists realized thatmutant alleles of a gene might be the consequence of alterations at different mutable sites within the gene. Theevidence that led to this idea was the finding of rare recombination events within several genes in Drosophila.
Theinitial observation came in 1940 from investigations by C. P. Oliver of a gene in the X chromosome of D.melanogaster known as lozenge (lz). Mutant alleles of lozenge are recessive, and their phenotypic effects include adisturbed arrangement of the facets of the compound eye and a reduction in the eye pigments. Numerous lz alleleswith distinguishable phenotypes are known.Females heterozygous for two different lz alleles, one in each homologous chromosome, have lozenge eyes. Whensuch heterozygous females were crossed with males that had one of the lz mutant alleles in the X chromosome, andlarge numbers of progeny were examined, flies with normal eyes were occasionally found.
For example, one crosscarried out wasin which lzBS and lzg are mutant alleles of lozenge, and ct (cut wing) and v (vermilion eye color) are genetic markersthat map 7.7 units to the left and 5.3 units to the right, respectively, of the lozenge locus. From this cross, 134males and females with wildtype eyes were found among more than 16,000 progeny, for a frequency of 8 × 10-3.These exceptional progeny might have resulted from a reverse mutation of one or of the other of the mutantlozenge alleles to lz+, but the observed frequency, though small, was much greater than the known frequencies ofreverse mutations.
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