Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 29
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However, a mechanism called tolerance prevents an organism from producing antibodiesagainst its own antigens. This mechanism ensures that A antigen or B antigen elicits antibody production only inpeople whose own red blood cells do not contain A or B, respectively. The end result:People of blood type O make both anti-A and anti-B antibodies; those of blood type A make anti-Bantibodies; those of blood type B make anti-A antibodies; and those of blood type AB make neither type ofantibody.The antibodies found in the blood fluid of people with each of the ABO blood types are shown in the fourthcolumn in Table 2.3.
The clinical significance of the ABO blood groups is that transfusion of blood containing A orB red-cell antigens into persons who make antibodies against them results in an agglutination reaction in which thedonor red blood cells are clumped. In this reaction, the anti-A anti-body will agglutinate red blood cells of eitherblood type A or blood type AB, because both carry the A antigen (Figure 2.30). Similarly, anti-B antibody willagglutinate red blood cells of either blood type B or blood type AB. When the blood cells agglutinate, many bloodvessels are blocked, and the recipient of the transfusion goes into shock and may die. Incompatibility in the otherdirection, in which the donor blood contains antibodies against the recipient's red blood cells, is usually acceptablebecause the donor's antibodies are diluted so rapidly that clumping is avoided.
The types of compatible bloodtransfusions are shown in the last two columns of Table 2.3. Note that a person of blood type AB can receive bloodfrom a person of any other ABO type; type AB is called a universal recipient. Conversely, a person of blood typeO can donate blood to a person of any ABO type; type O is called a universal donor.Incomplete Penetrance and Variable ExpressivityMonohybrid Mendelian ratios, such as 3 : 1 (or 1 : 2 : 1 when the heterozygote is intermediate), are not alwaysobserved even when a trait is determined by the action of a single recessive allele.
Regular ratios such as theseindicate that organisms with the same genotype also exhibit the same phenotype. Although the phenotypes oforganisms with a particular genotype are often very similar, this is not always the case—particularly in naturalpopulations in whichTable 2.3 Genetic control of the human ABO blood groupGenotypeAntigens presenton red blood cellsABO blood goupphenotypeAntibodies presentin blood fluidBlood types thatcan be toleratedin transfusionBlood types thatcan accept bloodfor transfusionI AI AAType AAnti-BA &OA & ABI AI OAType AAnti-BA&OA & ABIBIBBType BAnti-AB&OB & ABIBIOBType BAnti-AB&OB & ABI AI Ba&BType ABNeither anti-A noranti-BA, B, AB & OAB onlyI OI ONeither A nor BType OAnti-A & anti-BO onlyA, B, AB & OPage 71Figure 2.30Antibody against type-A antigen will agglutinate red blood cellsthat carry the type A antigen, whether or not they also carry thetype-B antigen.
Blood fluid containing anti-A antibody will agglutinatered blood cells of type A and type AB, but not red blood cells oftype B or type O.neither the matings nor the environmental conditions are under an experimenter's control. Variation in thephenotypic expression of a particular genotype may happen because other genes modify the phenotype or becausethe biological processes that produce the phenotype are sensitive to environmental conditions.The types of variable gene expression are usually grouped into two categories:• Variable expressivity refers to genes that are expressed to different degrees in different organisms.
For example,inherited genetic diseases in human beings are often variable in expression from one person to the next. One patientmay be very sick, whereas another with the same disease may be less severely affected. Variable expressivitymeans that the same mutant gene can result in a severe form of the disease in one person but a mild form inanother. The different degrees of expression often form a continuous series from full expression to almost noexpression of the expected phenotypic characteristics.• Incomplete penetrance means that the phenotype expected from a particular genotype is not always expressed.For example, a person with a genetic predisposition to lung cancer may not get the disease if he or she does notsmoke tobacco. A lack of gene expression may result from environmental conditions, such as in the example ofPage 72not smoking, or from the effects of other genes.
Incomplete penetrance is but an extreme of variable expressivity,in which the expressed phenotype is so mild as to be undetectable. The proportion of organisms whose phenotypematches their genotype for a given character is called the penetrance of the genotype. A genotype that is alwaysexpressed has a penetrance of 100 percent.Chapter SummaryInherited traits are determined by particulate elements called genes. In a higher plant or animal, the genes arepresent in pairs. One member of each gene pair is inherited from the maternal parent and the other member fromthe paternal parent.
A gene can have different forms owing to differences in DNA sequence. The different forms ofa gene are called alleles. The particular combination of alleles present in an organism constitutes its genotype. Theobservable characteristics of an organism constitute its phenotype. In an organism, if the two alleles of a gene pairare the same (for example, AA or aa), then the genotype is homozygous for the A or a allele; if the alleles aredifferent (Aa), then the genotype is heterozygous.
When the phenotype of a heterozygote is the same as that of oneof the homozygous genotypes, the allele that is expressed is called dominant and the hidden allele is calledrecessive.In genetic studies, the organisms produced by a mating constitute the F1 generation. Matings between members ofthe F1 generation produce the F2 generation. In a cross such as AA × aa, in which only one gene is considered (amonohybrid cross), the ratio of genotypes in the F2 generation is 1 dominant homozygote (AA) : 2 heterozygotes(Aa) : 1 recessive homozygote (aa).
The phenotypes in the F2 generation appear in the ratio 3 dominant: 1recessive. The Mendelian ratios of genotypes and phenotypes result from segregation in gamete formation, whenthe members of each allelic pair segregate into different gametes, and random union of gametes in fertilization.The processes of segregation, independent assortment, and random union of gametes follow the rules ofprobability, which provide the basis for predicting outcomes of genetic crosses. Two basic rules for combiningprobabilities are the addition rule and the multiplication rule.
The addition rule applies to mutually exclusiveevents; it states that the probability of the realization of either one or the other of two events equals the sum of therespective probabilities. The multiplication rule applies to independent events; it states that the probability of thesimultaneous realization of both of two events is equal to the product of the respective probabilities. In someorganisms—for example, human beings—it is not possible to perform controlled crosses, and genetic analysis isaccomplished through the study of several generations of a family tree, called a pedigree. Pedigree analysis is thedetermination of the possible genotypes of the family members in a pedigree and of the probability that anindividual member has a particular genotype.The complementation test is the functional definition of a gene.
Two recessive mutations are considered alleles ofdifferent genes if a cross between the homozygous recessives results in nonmutant progeny. Such alleles are said tocomplement one another. On the other hand, two recessive mutations are considered alleles of the same gene if across between the homozygous recessives results in mutant progeny. Such alleles are said to fail to complement.For any group of recessive mutations, a complete complementation test entails crossing the homozygous recessivesin all pairwise combinations.Multiple alleles are often encountered in natural populations or as a result of mutant screens.
Multiple allelesmeans that more than two alternative forms of a gene exist. Examples of large numbers of alleles include the genesused in DNA typing and the self-sterility alleles in some flowering plants. Although there may be multiple allelesin a population, each gamete can carry only one allele of each gene, and each organism can carry at most twodifferent alleles of each gene.Dihybrid crosses differ in two genes—for example, AA BB × aa bb.
The phenotypic ratios in the dihybrid F2 are 9 :3 : 3 : 1, provided that both the A and the B alleles are dominant and that the genes undergo independentassortment. The 9 : 3 : 3 : 1 ratio can be modified in various ways by interaction between the genes (epistasis).Different types of epistasis may result in dihybrid ratios such as 9 : 7, 12 : 3 : 1, 13 : 3, 9 : 4 : 3, and 9 : 6 : 1.In heterozygous genotypes, complete dominance of one allele over the other is not always observed. In most cases,a heterozygote for a wildtype allele and a mutant allele encoding a defective gene product will produce less geneproduct than in the wildtype homozygote.
If the phenotype is determined by the amount of wildtype gene productrather than by its mere presence, the heterozygote will have an intermediate phenotype. This situation is calledincomplete dominance. Codominance means that both alleles in a heterozygote are expressed, so the heterozygousgenotype exhibits the phenotypic characteristics of both homozygous genotypes. Codominance is exemplified bythe IA and IB alleles in persons with blood group AB.
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