Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 45
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In this chapter, we will examine phenomena that cause deviations from thisexpected ratio.In his early experiments with Drosophila, Morgan found mutations in each of several X-linked genes that providedideal materials for studying the inheritance of genes in the same chromosome. One of these genes, with alleles w+and w, determined normal red eye color versus white eyes, as discussed in Chapter 3; another such gene, with thealleles m+ and m, determined whether the size of the wings was normalPage 125or miniature.
The initial cross was between females with white eyes and normal wings and males with red eyes andminiature wings. We will use the slash in this instance to help us follow these X-linked traits:The resulting F1 progeny consisted of wildtype females and white-eyed, nonminiature males. When these werecrossed,the female progeny consisted of a 1 : 1 ratio of red : white eyes (all were nonminiature), and the male progeny wereas follows:Because each male receives his X chromosome from his mother, the phenotype reveals the genotype of the Xchromosome that he inherited. The results of the experiment show a great departure from the 1 : 1 : 1 : 1 ratio ofthe four male phenotypes expected with independent assortment.
If genes in the same chromosome tended toremain together in inheritance but were not completely linked, this pattern of deviation might be observed. In thiscase, the combinations of phenotypic traits in the parents of the original cross (parental phenotypes) were present in428/644 (66.5 percent) of the F2 males, and nonparental combinations (recombinant phenotypes) of the traits werepresent in 216/644 (33.5 percent).
The 33.5 percent recombinant X chromosomes is called the frequency ofrecombination, and it should be contrasted with the 50 percent recombination expected with independentassortment.The recombinant X chromosomes w+ m+ and w m result from crossing-over in meiosis in F1 females. In thisexample, the frequency of recombination between the linked w and m genes was 33.5 percent, butFigure 4.1Alleles of genes in different chromosomes undergo independentassortment. The pairs of homologous chromosomes segregate atrandom with respect to one another, so an A-bearing chromosome is aslikely to go to the same anaphase pole with a B-bearing chromosome aswith a b-bearing chromosome.
The result is that each possible combinationof chromatids is equally likely among the gametes: 1/4 each forA B, A b, a B, and a b.with other pairs of linked genes it ranges from near 0 to 50 percent. Even genes in the same chromosome canundergo independent assortment (frequency of recombination equal to 50 percent) if they are sufficiently far apart.This implies the following principle:Genes with recombination frequencies smaller than 50 percent are present in the same chromosome(linked).
Two genes that undergo independent assortment, indicated by a recombination frequency equal to50 percent, either are in nonhomologous chromosomes or are located far apart in a single chromosome.Geneticists use a notation for linked genes that has the general form w+ m/w m+. This notation is a simplification ofa more descriptive but cumbersome form:In this form of notation, the horizontal line separates the two homologous chromosomes in which the alleles of thegenes arePage 126located. The linked genes in a chromosome are always written in the same order for consistency.
In the system ofgene notation used for Drosophila, and in a similar system used for other organisms, this convention makes itpossible to indicate the wildtype allele of a gene with a plus sign in the appropriate position. For example, thegenotype w m+/w+m can be written without ambiguity as w +/+ m.''A genotype that is heterozygous for each of two linked genes can have the alleles in either of two possibleconfigurations, as shown in Figure 4.2 for the w and m genes. In one configuration, called the trans, or repulsion,configuration, the mutant alleles are in opposite chromosomes, and the genotype is written as w+/+ m (Figure4.2A).
In the alternative configuration, called the cis, or coupling, configuration, the mutant alleles are present inthe same chromosomes, and the genotype is written as w m/+ +.Morgan's study of linkage between the white and miniature alleles began with the trans configuration. He alsostudied progeny from the cis configuration of the w and m alleles, which results from the cross of white, miniaturefemales with red, non-miniature males:In this case, the F1 females were phenotypically wildtype double heterozygotes, and the males had white eyes andminiature wings. When these F1 progeny were crossed,Figure 4.2There are two possible configurations of the mutant alleles in a genotypethat is heterozygous for both mutations.
(A) The trans, or repulsion,configuration has the mutant alleles on opposite chromosomes. (B) The cis, orcoupling, configuration has the mutant alleles on the same chromosome.they produced the following progeny:Compared to the preceding experiment with w and m, the frequency of recombination between the genes isapproximately the same: 37.7 percent versus 33.5 percent. The difference is within the range expected fromrandom variation from experiment to experiment. However, in this case, the phenotypes constituting the parentaland recombinant classes of offspring are reversed. They are reversed because the original parents of the F1 femalewere different. In the first cross, the F1 female was the trans double heterozygote (w +/+ m); in the second cross,the F1 female had the cis configuration (w m/+ +).
The repeated finding of equal recombination frequencies inexperiments of this kind leads to the following conclusion:Recombination between linked genes takes place with the same frequency whether the alleles of the genesare in the trans configuration or in the cis configuration; it is the same no matter how the alleles arearranged.The recessive allele y of another X-linked gene in Drosophila results in yellow body color instead of the usual graycolor determined by the y+ allele. When white-eyed females were mated with males having yellow bodies, and thewildtype F1 females were testcrossed with yellow-bodied, white-eyed males,Page 127the progeny wereIn a second experiment, yellow-bodied, white-eyed females were crossed with wildtype males, and the F1 wildtypefemales and F1 yellow-bodied, white-eyed males were intercrossed:In this case, 98.6 percent of the F2 progeny had parental phenotypes and 1.3 percent had recombinant phenotypes.The parental and recombinant phenotypes were reversed in the reciprocal crosses, but the recombination frequencywas virtually the same.
Females with the trans genotype y +/+ w produced about 1.4 percent recombinant progeny,carrying either of the recombinant chromosomes y w or + +; similarly, females with the cis genotype y w/ + +produced about 1.4 percent recombinant progeny, carrying either of the recombinant chromosomes y + or + w.However, the recombination frequency was much lower between the genes for yellow body and white eyes thanbetween the genes for white eyes and miniature wings (1.4 percent versus about 35 percent). These and otherexperiments have led to the following conclusions:• The recombination frequency is a characteristic of a particular pair of genes.• Recombination frequencies are the same in cis (coupling) and trans (repulsion) heterozygotes.In experiments with other genes, Morgan also discovered that Drosophila is unusual in that recombination does nottake place in males.
Although it is not known how (or why) crossing-over is prevented in males, the result of theabsence of recombination in Drosophila males is that all alleles located in a particular chromosome show completelinkage in the male. For example, the genes cn (cinnabar eyes) and bw (brown eyes) are both in chromosome 2 butare so far apart that, in females, there is 50 percent recombination.
Thus the crossyields progeny of genotype + +/cn bw and cn bw/cn bw (the nonrecombinant types) as well as cn +/cn bw and +bw/cn bw (the recombinant types) in the proportions 1 : 1 : 1 : 1. However, because there is no crossing-over inmales, the reciprocal crossyields progeny only of the nonrecombinant genotypes + + /cn bw and cn bw/cn bw in equal proportions. Theabsence of recombination in Drosophila males is a convenience often made use of in experimental design; asshown in the case of cn and bw, all the alleles present in any chromosome in a male must be transmitted as a group,without being recombined with alleles present in the homologous chromosome.
The absence of crossing-over inDrosophila males is atypical; in most other animals and plants, recombination takes place in both sexes.4.2—Genetic MappingThe linkage of the genes in a chromosome can be represented in the form of a genetic map, which shows the linearorder of the genes along the chromosome with the distances between adjacent genes proportional to the frequencyof recombination between them. A genetic map is also called a linkage map or a chromosome map. The conceptof genetic mapping was first developed by Morgan's student, Alfred H. Sturtevant, in 1913. The early geneticistsunderstood that recombination between genes takes place by an exchange ofPage 128segments between homologous chromosomes in the process now called crossing-over.
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