Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 47
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. . . These results form a new argument in favor of thechromosome view of inheritance, since they strongly indicate that the factors investigated are arranged in a linearseries.Source: Journal of Experimental Zoology 14: 43–59When adjacent chromosome regions separating linked genes are sufficiently short that multiple crossovers are notformed, the recombination frequencies (and hence the map distances) between the genes are additive. Thisimportant feature of recombination, and also the logic used in genetic mapping, is illustrated by the example inFigure 4.7. The genes are all in the X chromosome of Drosophila: y (yellow body), rb (ruby eye color), and cv(shortened wing crossvein). The recombination frequency between genes y and rb is 7.5 percent, and that betweenrb and cv is 6.2 percent.
The genetic map might be any one of three possibilities, depending on which gene is in themiddle (y, cv, or rb). Map A, which has y in the middle, can be excludedPage 132Figure 4.7In Drosophila, the genes y (yellow body) and rb (ruby eyes) have a recombination frequency of 7.5percent, and rb and cv (shortened wing crossvein) have a recombination frequency of 6.2 percent.There arethree possible genetic maps, depending on whether y is in the middle (A), cv is in the middle(B), or rb is in the middle (C). Map A can be excluded because it implies that rb and y are closerthen rb and cv, whereas the observed recombination frequency between rb and y is larger than thatbetween rb and cv. Maps B and C are compatible with the data given.because it implies that the recombination frequency between rb and cv should be greater than that between rb andy, and this contradicts the observed data.Maps B and C are both consistent with the recombination frequencies. They differ in their predictions regarding therecombination frequency between y and cv.
In map B the predicted distance is 1.3 map units, whereas in map C thepredicted distance is 13.3 map units. In reality, the observed recombination frequency between y and cv is 13.3percent. Map C is therefore correct.There are actually two genetic maps corresponding to map C. They differ only in whether y is placed at the left orthe right. One map isThese two ways of depicting the genetic map are completely equivalent.A genetic map can be expanded by this type of reasoning to include all the known genes in a chromosome; thesegenes constitute a linkage group.
The number of linkage groups is the same as the haploid number ofchromosomes of the species. For example, cultivated corn (Zea mays) has ten pairs of chromosomes and tenlinkage groups. A partial genetic map of chromosome 10 is shown in Figure 4.8, along with the dramaticphenotypes shown by some of the mutants. The ears of corn in the two photographs (Figure 4.8C and 4.8F)demonstrate the result of Mendelian segregation. The photograph in Figure 4.8C shows a 3 : 1 segregation ofyellow : orange kernels produced by the recessive orange pericarp-2 (orp-2) allele in a cross between twoheterozygous genotypes. The ear in the photograph in Figure 4.8F shows a 1 : 1 segregation of marbled : whitekernels produced by the dominant allele R1-mb in a cross between a heterozygous genotype and a homozygousnormal.Crossing-overThe orderly arrangement of genes represented by a genetic map is consistent with the conclusion that each geneoccupies a well-defined site, or locus, in the chromosome, with the alleles of a gene in a heterozygote occupyingcorresponding locations in the pair of homologous chromosomes.
Crossing-over, which is brought about by aphysical exchange of segmentsPage 133Figure 4.8Genetic map of chromosome 10 of corn, Zea mays. The map distance to each gene is given in standardmap units (centimorgans) relative to a position 0 for the telomere of the short arm (lower left).Mutations in the gene lesion-6 (les6), result in many small to medium-sized, irregularly spaced, discoloredspots on the leaf blade and sheath;(A) shows the phenotype of a heterozygote for Les6, a dominant allele.Mutations in the gene oil yellow-1 (oy1) result in a yellow-green plant. In (B), the plant in front isheterozygous for the dominant allele Oy1: behind is a normal plant.
The orp2 allele is a recessiveexpressed as orange pericarp, a maternal tissue that surrounds the kernels; (C) shows the segregationof orp2 in a cross between two heterozygous genotypes, yielding a 3 : 1 ratio of yellow : orangeseeds. The gene znl is zebra necrotic-1, in which dying tissue appears in transverse leaf bands;in (D), the left leaf is homozygous znl, the right leaf wildtype.
Mutations in the gene teopod-2(tp2) result in many small, partially podded ears and a simple tassle; one of the ears in a plantheterozygous for the dominant allele Tp2 is shown (E). The mutation R1-mb isan allele of the rl gene resulting in red or purple color in the aleurone layer of the seed; (F) showsthe marbeled color in kernels of an ear segregating forR1-mb.[Photographs courtesy of M.
G. Neuffer; genetic map courtesy of E. H. Coe.]Page 134that results in a new association of genes in the same chromosome, has the following features:1. The exchange of segments between parental chromatids takes place in the first meiotic prophase, after thechromosomes have duplicated. The four chromatids (strands) of a pair of homologous chromosomes are closelysynapsed at this stage. Crossing-over is a physical exchange between chromatids in a pair of homologouschromosomes.2. The exchange process consists of the breaking and rejoining of the two chromatids, resulting in the reciprocalexchange of equal and corresponding segments between them (see Figure 4.3).3.
The sites of crossing-over are more or less random along the length of a chromosome pair. Hence the probabilityof crossing-over between two genes increases as the physical distance between the genes along the chromosomebecomes larger. This principle is the basis of genetic mapping.The demonstration that crossing-over (as detected by the recombination of two heterozygous markers) is associatedwith a physical exchange of segments between homologous chromosomes was made possible by the discovery oftwo structurally altered chromosomes that permitted the microscopic recognition of parental and recombinantchromosomes.
In 1931, Curt Stern discovered two X chromosomes of Drosophila that had undergone structuralchanges that made them distinguishable from each other and from a normal X chromosome. He used thesestructurally altered X chromosomes in an experiment that provided one of the classical proofs of the physical basisof crossing-over (Figure 4.9). One of the altered X chromosomes was missing a segment that had become attachedto chromosome 4. This altered X chromosome could be identified by its missing terminal segment. The secondaberrant X chromosome had a small piece of a Y chromosome attached as a second arm. The mutant alleles car(abbreviated c, a recessive allele resulting in carnation eye color instead of wildtype red) and B (a dominant alleleresulting in bar-shaped eyes instead of round) were present in the first altered X chromosome, and the wild-typealleles of these genes were in the second altered X.
Females with the two structurally and genetically marked Xchromosomes were mated with males having a normal X that carried the recessive alleles of the genes (Figure 4.9).In the progeny from this cross, flies with parental or recombinant combinations of the phenotypic traits wererecognized by their eye color and shape, and their chromosomal makeup could be determined by microscopicexamination of the offspring they produced in a testcross. In the genetically recombinant progeny from the cross,the X chromosome had the morphology that would be expected if recombination of the genes were accompaniedby an exchange that recombined the chromosome markers; that is, the progeny with wildtype (red), bar-shapedeyes had an X chromosome with a missing terminal segment and the attached Y arm; similarly, progeny withcarnation-colored, round eyes had a structurally normal X chromosome with no missing terminus and no Y arm.
Asexpected, the nonrecombinant progeny were found to have an X chromosome morphologically identical with onein their mothers.Crossing-over Takes Place at the Four-Strand Stage of MeiosisSo far we have asserted, without citing experimental evidence, that crossing-over takes place in meiosis after thechromosomes have duplicated, at the stage when each bivalent has four chromatid strands. One experimental proofthat crossing-over takes place after the chromosomes have duplicated came from a study of laboratory stocks of D.melanogaster in which the two X chromosomes in a female are joined to a common centromere to form an aberrantchromosomes called an attached-X, or compound-X, chromosome.
The normal X chromosome in Drosophila hasa centromere almost at the end of the chromosome, and the attachment of two of these chromosomes to a singlecentromere results in a chromosome with two equal arms, each consisting of a virtually complete X. Females witha compound-X chro-Page 135Figure 4.9(A) Diagram of a cross in which the two X chromosomes in a Drosophila female are morphologically distinguishablefrom each other and from a normal X chromosome.
One X chromosome has a missing terminal segment,and the other has a second arm consisting of a fragment of the Y chromosome. (B) Result of the cross. The carnationoffspring contain a structurally normal X chromosome, and the bar offspring contain an X chromosome with bothmorphological markers.The result demonstrates that genetic recombination between marker genes isassociated with physical exchange between homologous chromosomes.
Segregation of the missingterminal segment of the X chromosome, which is attached to chromosome 4, is not shown.mosome usually contain a Y chromosome as well, and they produce two classes of viable offspring: females whohave the maternal compound-X chromosome along with a paternal Y chromosome, and males with the maternal Ychromosome along with a paternal X chromosome (Figure 4.10). Attached-X chromosomes are frequently used tostudy X-linked genes in Drosophila because a male carrying any X-linked mutation, when crossed with anattached-X female, produces sons who also carry the mutation and daughters who carry the attached-Xchromosome. In matings with attached-X females, therefore, the inheritance of an X-linked gene in the male passesfrom father to son to grandson, and so forth, which is the opposite of usual X-linked inheritance.In an attached-X chromosome in which one X carries a recessive allele and the other carries the wildtypenonmutant allele,Page 136Figure 4.10Attached-X (compound-X) chromosomes in Drosophila.
(A) A structurallynormal X chromosome in a female. (B) An attached-X chromosome,with the long arms of two normal X chromosomes attached to acommon centromere. (C) Typical attached-X females alsocontain a Y chromosome. (D) Outcome of a cross between an attached-X femaleand a normal male.
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