Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 26
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Because no X is produced,there can be no Y, and without Y there can be no anthocyanin, and so homozygous alb alb results in white flowers.The alb allele is recessive, because in the heterozygous genotype Alb alb, the wildtype Alb allele codes for afunctional enzyme for the first step in the pathway, and so the pathway is not blocked.Mutant alleles of the C gene block the second step in the pathway. In this case, an inactive enzyme is produced notonly in the homozygous genotypes cc, 2c2, and c4c4, but also in the genotypes cc2, cc4, and c2c.4 In the last threegenotypes, each mutant allele encodes a different (but still inactive) form of the enzyme, so the pathway is blockedat step 2, and the color of the flowers is white.
The c, c2, and c4 alleles are all in the same complementation groupPage 60(they fail to complement one another) because they all encode inactive forms of the same enzyme.A similar situation holds for mutations in the P gene. The wildtype P allele encodes the enzyme for the final step inthe pathway to anthocyanin. Any of the genotypes pp, pp3, and p3p3 lacks a functional form of the enzyme, whichblocks the pathway at the last step and results in white flowers. The alleles p and p3 are in the samecomplementation group because they are both mutations in the P gene.Multiple AllelesThe C and P genes in Figure 2.22 also illustrate the phenomenon of multiple alleles, in which there are more thantwo allelic forms of a given gene. Because the wildtype form of each gene also counts as an allele, there are twoalleles of the Alb gene (Alb and alb), four alleles of the C gene (C, c, c2, and c4,) and three alleles of the P gene (P,p,and p3).When a complementation test reveals that two independent mutations are alleles of the same gene, one does notknow whether the mutant alleles have identical nucleotide sequences in the DNA.
Recall from Chapter 1 that, atthe level of DNA, a gene is a sequence of nucleotides that specifies the sequence of amino acids in a protein. Eachnucleotide contains a base, either A (adenine), T (thymine), G (guanine), or C (cytosine), so a gene of n nucleotidescan theoretically mutate at any of the positions to any of the three other nucleotides. The number of possible singlenucleotide differences in a gene of length n is therefore 3 × n; each of these DNA sequences, if it exists in thepopulation, is an allele.
When n = 5000, for example, there are potentially 15,000 alleles (not counting any of thepossibilities with more than one nucleotide substitution). Most of the potential alleles may not actually exist at anyone time, but many of them may be present in any population. The following rules govern the number of alleles.• A gamete may contain only one allele of each gene.• Any particular organism or cell may contain up to two different alleles.• A population of organisms may contain any number of allelesMany genes have multiple alleles. For example, the human blood groups designated A, B, O, or AB are determinedby three types of alleles denoted IA, IB, and IO, and the blood group of any person is determined by the particularpair of alleles present in his or her genotype.
(Actually, there are two slightly different variants of the IAFigure 2.22Biochemical pathway for the synthesis of the purple pigment anthocyanin from a colorless precursor and colorlessintermediates X and Y. Each step (arrow) in the pathway is a biochemical conversion that requires anenzyme encoded in the wildtype allele of the gene indicated.Page 61allele, so four alleles can be distinguished in this case.)In modern genetics, multiple alleles are encountered in two major settings.
One is in genetic analysis when amutant screen potentially yields two or more mutant alleles of each of a large number of genes. For example, in theearly 1980s, mutant screens were carried out in Drosophila to obtain new recessive mutations that blockedembryonic development and so led to the death of homozygous recessive embryos. The screens resulted in theidentification of approximately 18,000 such mutations, the study of which ultimately earned a 1995 Nobel Prize forChristiane Nüsslein-Volhard and Eric Wieschaus (they shared the prize with Edward B.
Lewis, who had alreadydone pioneering work in the genetics of Drosophila development).Geneticists also encounter multiple alleles in studies of natural populations of organisms. In most populations,including the human population, each gene may have many alleles that differ slightly in nucleotide sequence. Mostof these alleles, even though they differ in one or more nucleotides in the DNA sequence, are able to carry out thenormal function of the gene and produce no observable difference in phenotype.In human populations, it is not unusual for a gene to have many alleles. Genes used in DNA typing, such as thoseemployed in criminal investigations, usually have multiple alleles in the population.
For each of these genes, anyperson can have no more than two alleles, but often there are 20 or more alleles in the population as a whole.Hence, any two unrelated people are not likely to have the same genotype, especially if several different genes,each with multiple alleles, are examined. Similarly, in the inherited recessive diseases cystic fibrosis andphenylketonuria, more than 200 different defective alleles of each gene have been identified in studies of affectedchildren throughout the world. The "normal" form of each gene also exists in many alternative forms.
Indeed, formost genes in most populations, the "normal" or "wildtype" allele is not a single nucleotide sequence but rather aset of different nucleotide sequences, each capable of carrying out the normal function of the gene.In some cases, the multiple alleles of a gene exist merely by chance and reflect the history of mutations that havetaken place in the population and the dissemination of these mutations among population subgroups by migrationand interbreeding. In other cases, there are biological mechanisms that favor the maintenance of a large number ofalleles. For example, genes that control self-sterility in certain flowering plants can have large numbers of allelictypes.
This type of self-sterility is found in species of red clover that grow wild in many pastures. The self-sterilitygenes prevent self-fertilization because a pollen grain can undergo pollen tube growth and fertilization only if itcontains a self-sterility allele different from either of the alleles present in the flower on which it lands. In otherwords, a pollen grain containing an allele already present in a flower will not function on that flower. Because allpollen grains produced by a plant must contain one of the self-sterility alleles present in the plant, pollen cannotfunction on the same plant that produced it, and self-fertilization cannot take place.
Under these conditions, anyplant with a new allele has a selective advantage, because pollen that contains the new allele can fertilize allflowers except those on the same plant. Through evolution, populations of red clover have accumulated hundredsof alleles of the selfsterility gene, many of which have been isolated and their DNA sequences determined. Manyof the alleles differ at multiple nucleotide sites, which implies that the alleles in the population are very old.2.6—Modified Dihybrid Ratios Caused by EpistasisIn Figure 2.22 we saw how the products of several genes may be necessary to carry out all the steps in abiochemical pathway. In genetic crosses in which two mutations that affect different steps in a single pathway areboth segregating, the typical F2 dihybrid ratio of 9 : 3 : 3 : 1 is not observed.
One way in which the ratio may bemodified is illustrated by the interaction of the C,Page 62Figure 2.23A cross showing epistasis in the determination of flower color insweet peas. Formation of the purple pigment requires the dominantallele of both the C and P genes. With this type of epistasis, the dihybridF2 ratio is modified to 9 purple : 7 white.c and P, p allele pairs affecting flower coloration.
Figure 2.23 shows a cross between the homozygous mutants ppand cc. The phenotype of the plants in the F1 generation is purple flowers; complementation is observed becausethe p and c mutations are in different genes. Self-fertilization of the F1 plants (indicated by the encircled cross sign)results in the F2 progeny genotypes shown in the Punnett square. Because only the progeny with at least one Callele and at least one P allele have purple flowers and all the rest have white flowers, the ratio of purple flowers towhite flowers in the F2 generation is 9 : 7.Any type of gene interaction that results in the F2 dihybrid ratio of 9 : 3 : 3 : 1 being modified into some other ratiois called epistasis. For a trait determined by the interaction of two genes, each with a dominant allele, there are onlya limited number of ways in which the 9:3:3:1 dihybrid ratio can be modified. The possibilities are illustrated inFigure 2.24.
In part A are the genotypes produced in the F2 generation by independent assortment and the ratios inwhich the genotypes occur. In the absence of epistasis, the F2 ratio of phenotypes is 9 : 3 : 3 : 1. The possiblemodified ratios are shown in part B of the figure. In each row, the color coding indicates phenotypes that areindistinguishable because of epistasis, and the resulting modified ratio is given. For example, in the modified ratioat the bottom, the phenotypes of the "3:3:1" classes are indistinguishable, resulting in a 9 : 7 ratio.
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