Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 28
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(A) Relative amounts of starchbranching enzyme I(SBEI); the enzyme level in the heterozygous genotype is about halfway between thelevels in the homozygous genotypes. (B) Size and shape of the microscopic starchgrains; the heterozygote is intermediate. (C) Effect on shape of mature seeds; for seedshape, W is dominant over w.Page 66show simple dominance for some traits but not others.
The general principle illustrated in Figure 2.25 is:The phenotype consists of many different physical and biochemical attributes, and dominance may beobserved for some of these attributes and not for others. Thus dominance is a property of a pair of alleles inrelation to a particular attribute of phenotype.Amorphs, Hypomorphs, and Other Types of MutationsWe have seen that a wildtype allele can potentially undergo mutation at any of a large number of nucleotide sites inthe DNA, resulting in multiple alleles of a gene.
In a series of multiple alleles, some alleles may have a moredrastic effect on the phenotype than others. For example, one mutant allele may render the corresponding enzymecompletely inactive, whereas another mutant allele may impair the enzyme in such a way as to cause only a partialloss of enzyme activity. Geneticists sometimes classify mutations according to the severity of their effects.A mutation such as Mendel's wrinkled mutation, which encodes an inactive form of the SBEI enzyme, is oftencalled an amorph. At the molecular level, an amorphic mutation may result from an amino acid replacement thatinactivates the enzyme or even from a deletion of the gene so that no enzyme is produced. A mutation that reducesthe enzyme level, but does not eliminate it, is called a hypomorph.
Hypomorphic mutations typically result fromamino acid replacements that impair enzyme activity or that prevent the enzyme from being produced at the normallevel. As the prefix hyper implies, a hypermorph produces a greater-than-normal enzyme level, typically becausethe mutation changes the regulation of the gene in such a way that the gene product is overproduced.Relative to their effects on the protein product of the gene they affect, most mutations can be classified as amorphs,hypomorphs, or hypermorphs. They result in none, less, or more of the enzyme activity produced by the wildtype,nonmutant allele. But other types of mutations also arise.
A neomorph is a type of mutation that qualitativelyalters the action of a gene. For example, a neomorph may cause a gene to become active in a type of cell or tissuein which the gene is not normally active. Or a neomorph can result in the expression of a gene in development at atime during which the wildtype gene is not normally expressed.
Neomorphic mutations in a Drosophila gene calledeyeless, which cause the wildtype gene product to be expressed in non-eye-forming tissues, can result in thedevelopment of parts of compoundFigure 2.26Ecoptic expression of the wildtype allele of the eyeless gene in Drosophilaresult's in misplaced eye tissue. (A) An adult head in which both antennaeform eye structures.
(B) A wing with eye tissue growing out from it. (C)A single antenna in which most of the third segment consists of eye tissue.(D) Middle leg with an eye outgrowth at the base of the tibia.[Courtesy of G. Halder and W. J. Gehring. From G. Halder, P. Callaerts,and W. J. Gehring, Science 1995. 267: 1788.]Page 67eyes, complete with eye pigments, in abnormal locations.
The locations can be anywhere the wildtype eyeless geneis expressed, including on the legs or mouthparts, in the abdomen, or on the wings (Figure 2.26). Expression of awildtype gene in an abnormal location is called ectopic expression.Another type of mutation is an antimorph, whose mutant product antagonizes the normal product of the gene.
Insome cases this occurs through an amino acid replacement that causes the mutant protein to combine with thewildtype protein into an inactive complex. These various terms for mutations were coined by Herman J. Muller in1931. Muller also discovered that mutations can be caused by x rays (Chapter 13). Many x-ray-induced mutationsare associated with major disruptions or rearrangements of the DNA sequence, which result in unusual types ofpatterns of expression of the affected genes. Muller's terms were useful for describing such mutations, and theyhave come into widespread use for discussing other types of mutations as well.Incomplete DominanceWhen the phenotype of the heterozygous genotype lies in the range between the phenotypes of the homozygousgenotypes, there is said to be incomplete dominance.
Most genes code for enzymes, and each allele in a genotypeoften makes its own contribution to the total level of the enzyme in the cell or organism. In such cases, thephenotype of the heterozygote falls in the range between the phenotypes of the corresponding homozygotes, asillustrated in Figure 2.27.
There is no settled terminology for the situation: the terms incomplete dominance, partialdominance, and semidominance are all in use.A classical example of incomplete dominance concerns flower color in the snapdragon Antirrhinum (Figure 2.28).In wildtype flowers, a red type of anthocyanin pigment is formed by a sequence of enzymatic reactions. A wildtypeenzyme, encoded by the I allele, is limiting to the rate of the overall reaction, so the amount of red pigment isdetermined by the amount of enzyme that the I allele pro-Figure 2.27Levels of phenotypic expression in heterozygotes with complete dominanceand with incomplete dominance.duces.
The alternative i allele codes for an inactive enzyme, and ii flowers are ivory in color. Because the amountof the critical enzyme is reduced in Ii heterozygotes, the amount of red pigment in the flowers is reduced also, andthe effect of the dilution is to make the flowers pink.The result of Mendelian segregation is observed directly when snapdragons that differ in flower color are crossed.For example, a cross between plants from a true-breeding red-flowered variety and a true-breeding ivory-floweredvariety results in F1 plants with pink flowers. In the F2 progeny obtained by self-pollination of the F1 hybrids, oneexperiment resulted in 22 plants with red flowers, 52 with pink flowers, and 23 with ivory flowers.
The numbersagree fairly well with the Mendelian ratio of 1 dominant homozygote : 2 heterozygotes : 1 recessive homozygote.In agreement with the predictions from simple Mendelian inheritance, the red-flowered F2 plants produced onlyred-flowered progeny, the ivory-flowered plants produced only ivory-flowered progeny, and the pink-floweredplants produced red, pink, and ivory progeny in the proportions 1/4 red: 1/2 pink : 1/4 ivory.Incomplete dominance is often observed when the phenotype is quantitativePage 68Figure 2.28Absence of dominance in the inheritance of flower color in snapdragons.rather than discrete. A trait that is quantitative can be measured on a continuous scale; examples include height,weight, number of eggs laid by a hen, time of flowering of a plant, and amount of enzyme in a cell or organism.
Atrait that is discrete is all or nothing; examples include round versus wrinkled seeds, and yellow versus green seeds.With a phenotype that is quantitative, the measured value of a heterozygote usually falls in the range between thehomozygotes, and therefore there is incomplete dominance.Codominance and the Human ABO Blood GroupsA special term, codominance, refers to a situation in which the phenotype of a heterozygous genotype is a mixtureof the phenotypes of both of the corresponding homozygous genotypes.
In such cases, the heterozygous phenotypeis not intermediate between the homozygous genotypes (like pink snapdragons) but rather has the characteristics ofboth homozygous genotypes.What we mean by ''has the characteristics of both homozygous genotypes" is illustrated by one of the classicalexamples of codominance.
These are the alleles that determine the A, B, AB, and O human blood groups, whichwere discussed earlier in the context of multiple alleles. Blood type is determined by the types of polysaccharides(polymers of sugars) present on the surface of red blood cells. Two different polysaccharides, A and B, can beformed. Both are formed from a precursor substance that is modified by the enzyme product of either the IA or theIB allele. The gene products are transferase enzymes that attach either of two types of sugar units to the precursor(Figure 2.29). People of genotype IAIA produce red blood cells having only the A polysaccharide and are said tohave blood type A. Those of genotype IBIB have red blood cells with only the B polysaccharide and have bloodtype B.
Heterozygous IAIB people have red cells with both the A and B polysaccharides and have blood type AB.The IAIB genotype illustrates codominance, because the heterozygous genotype has the characteristics of bothhomozygous genotypes—in this case the presence of both the A and the B carbohydrate on the red blood cells.The third allele, IO, does not show codominance. It encodes a defective enzyme that leaves the precursorunchanged; neither the A nor the B type of polysaccharide is produced. Homozygous IOIO persons therefore lackboth the A and the B polysaccharides; they are said to have blood type O. In IAIO heterozygotes, pres-Page 69Figure 2.29The ABO antigens on the surface of human red blood cells are carbohydrates.
They are formed froma precursor carbohydrate by the action of transferase enzymes encoded by alleles of the I gene.Allele IO codes for an inactive enzyme and leaves the precursor unmodified. The unmodified formis called the H substance. The IA allele encodes an enzyme that adds N-acetylgalactosamine (purple)to the precursor. The IB allele encodes an enzyme that adds galactose (green) to the precursor.
Theother colored sugar units are N-acetylglucosamine (orange), and fucose (yellow). The sugar ringsalso have side groups attached to one or more of their carbon atoms; these are shown in thedetailed structures inside the box.ence of the IA allele results in production of the A polysaccharide; and in IBIO heterozygotes, presence of the IBallele results in production of the B polysaccharide. The result is that IAIO persons have blood type A and IBIOpersons have blood type B, so IO is recessive to both IA and IB. The genotypes and phenotypes of the ABO bloodgroup system are summarized in the first three columns of Table 2.3.The ABO blood groups are important in medicine because of the frequent need for blood transfusions.
A crucialfeature of the ABO system is that most human blood contains antibodies to either the A or the B polysaccharide.An antibody is a protein that is made by the immune system in response to a stimulating molecule called anantigen and is capable of binding to the antigen. An antibody is usually specific in that it recognizes only oneantigen. Some antibodies combine with antigen and form large molecular aggregates that may precipitate.Page 70Antibodies act in the body's defense against invading viruses and bacteria, as well as other cells, and help inremoving such invaders from the body. Although antibodies do not normally form without prior stimulation by theantigen, people capable of producing anti-A and anti-B antibodies do produce them. Production of these antibodiesmay be stimulated by antigens that are similar to polysaccharides A and B and that are present on the surfaces ofmany common bacteria.
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