Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 25
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Let us suppose that we were lucky enough to obtain four new mutations, in addition to the p and cmutations identified by the complementation test in Figure 2.19.How are we going to name these four new mutations? We can make no assumptions about the number of genesrepresented. All four could be recurrences of either p or c.
On the other hand, each of the four could be a newmutation in a different gene needed for flower color. For the moment, let us call the new mutations xl, x2, x3, andx4, where the ''x" does not imply a gene but rather that the mutation was obtained with x irradiation. Each mutationis recessive and was identified through the white flowers of the homozygous recessive F2 seeds (for example, x1x1).Now the complementation test is used to classify the "x" mutations into groups.Figure 2.20 shows that the results of a complementation test are typically reported in a triangular array of + and signs. The crosses that yield F1 progeny with the wildtype phenotype (in this case, purple flowers) are denoted witha + in the box where imaginary lines from the male parent and the female parent intersect. The crosses that yield F1progeny with the mutant phenotype (white flowers) are denoted with a - sign.
The + signs indicatecomplementation between the mutant alleles in the parents; the - signs indicate lack of complementation. Thebottom half of the triangle is unnecessary because the reciprocal of each cross produces F1 progeny with the samegenotype and phenotype as the cross that is shown. The diagonal elements are also unnecessary, because a crossbetween any two organisms carrying the identical mutation, forFigure 2.20Results of complementation tests among six mutant strains of peas, each homozygous for a recessive allele resultingin white flowers.
Each box gives the phenotype of the F1 progeny of a cross between the male parent whosegenotype is indicated in the far left column and the female parent whose genotype is indicated in the top row.Page 58example, xl xl × xl xl, must yield homozygous recessive xl xl progeny, which will be mutant.
As we have seen inFigure 2.19, complementation in a cross means that the parental strains have their mutations in different genes.Lack of complementation means that the parental mutations are in the same gene. The principle underlying thecomplementation test isThe Principle of Complementation: If two recessive mutations are alleles of the same gene, then thephenotype of an organism containing both mutations is mutant; if they are alleles of different genes, thenthe phenotype of an organism containing both mutations is wildtype (nonmutant).In interpreting complementation data such as those in Figure 2.20, we actually apply the principle the other wayaround. Examination of the phenotype of the F1 progeny of each possible cross reveals which of the mutations arealleles of the same gene:In a complementation test, if the combination of two recessive mutations results in a mutant phenotype,then the mutations are regarded as alleles of the same gene; if the combination results in a wildtypephenotype, then the mutations are regarded as alleles of different genes.A convenient way to analyze the data in Figure 2.20 is to arrange the alleles in a circle as shown in Figure 2.21A.Then, for each possible pair of mutations, connect the pair by a straight line if the mutations fail to complement(Figure 2.21B).
According to the principle of complementation, the lines must connect mutations that are alleles ofeach other because, in a complementation test, lack of complementation means that the mutations are alleles. Inthis example, mutation x3 is an allele of p, so x3 and p are different mutant alleles of the gene P. Similarly, themutations x2, x4, and c are different mutant alleles of the gene C. The mutation x1 does complement all of theothers.
It represents a third gene, different from P and C, that affects flower coloration.In an analysis like that in Figure 2.21, each of the groups of noncomplementing mutations is called acomplementation group. As we have seen, each complementation group defines a gene.A gene is defined experimentally as a set of mutations that make up one complementation group. Any pair ofmutations in such a group fail to complement one another and result in an organism with an observable mutantphenotype.The mutations in Figure 2.21 therefore represent three genes, a mutation in any one of which results in whiteflowers. The gene P is represented by the alleles p and x3; the gene C is represented by the alleles c, x2, and x4;and the allele xl represents a third gene different from either P or C.
Each gene coincides with one of thecomplementation groups.At this point in a genetic analysis, it is possible to rename the mutations to indicate which ones are true alleles.Because the p allele already had its name before the mutation screen was carried out to obtain more flower-colormutations, the new allele of p, x3, should be renamed to reflect its allelism with p. We might rename the x3mutation p3, for example, using the subscript to indicate that p3 arose independently of p. For similar reasons, wemight rename the x2 and x4 mutations c2 and c4 to reflect their allelism with the original c mutation and to conveytheir independent origins. The x1 mutation represents an allele of a new gene to which we can assign a namearbitrarily.
For example, we might call the mutation albus (Latin for white) and assign the x1 allele the new namealb. The wildtype dominant allele of alb, which is necessary for purple coloration, would then be symbolized asAlb or as alb+. The procedure of sorting new mutations into complementation groups and renaming them accordingto their allelism is an example of how geneticists identify genes and name alleles. Such renaming of alleles is thetypical manner in which genetic terminology evolves as knowledge advances.Why Does the Complementation Test Work?There is an old Chinese saying that the correct naming of things is the beginning of wisdom, and this is certainlytrue in the case of genes.
The proper renaming of thePage 59p, c, and alb mutations to indicate which mutations are alleles of the three genes is a wise way to create aterminology that i ndicates, for each possible genotype, what the phenotype will be with regard to flower color. Apurple flower requires the presence of at least one copy of each of the wildtype P, C, and Alb alleles. Any genotypethat contains two mutant alleles of P will have white flowers. These genotypes are pp, p3p3, and pp3.
Likewise, anygenotype that contains two mutant alleles of C will have white flowers. These genotypes are cc, c2c2, c4c4, cc2, cc4,and c2c4. Finally, any genotype that contains two mutant alleles of Alb will have white flowers. In this case there isonly one such genotype, alb alb.The biological reason why the screen for flower-color mutants yielded mutations in each of three genes is based onthe biochemical pathway by which the purple pigment is synthesized in the flowers. Examination of thebiochemical pathway also explains why the complementation test works.
The pathway is illustrated in Figure 2.22.The purple pigment anthocyanin is produced from a colorless precursor by way of two colorless intermediatecompounds denoted X and Y. Each arrow represents a "step" in the pathway, a biochemical conversion from onesubstance to the next along the way. Each step requires an enzyme encoded by the wildtype allele of the geneindicated at the top. The allele P, for example, codes for the enzyme required in the last step in the pathway, whichconverts intermediate compound Y into anthocyanin. If this enzyme is missing (or is present in an inactive form),the intermediate substance Y cannot be converted into anthocyanin.
The pathway is said to be "blocked" at thisstep. Although the block causes an increase in the concentration of Y inside the cell (because the precursor is stillconverted into X, and X into Y), no purple pigment is produced and the flower remains white.Each of the mutant alleles listed across the bottom of the pathway codes for an inactive form of the correspondingenzyme. Any genotype that is homozygous for any of the mutations fails to produce an active form of the enzyme.For example, because the mutant alb allele codes for an inactive form of the enzyme for the firstFigure 2.21A method for interpreting the results of complementation tests.
(A)Arrange the mutations in a circle. (B) Connect by a straight line anypair of mutations that fails to complement (that yields amutant phenotype); any pair of mutations so connected are alleles of thesame gene. In this example, there are three complementation groups,each of which represents a single gene needed for purple flowercoloration.step in the pathway, the genotype alb alb has no active enzyme for this step. In the homozygous mutant, thepathway is blocked at the first step, so the precursor is not converted to intermediate X.
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