Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 55
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Genetic variation of this type is called restriction fragment lengthpolymorphism (RFLP). Most species also have considerable genetic variation in which one allele differs from thenext according to the number of copies of a tandemly repeated DNA sequence it contains (variable number oftandem repeats, or VNTR). Many of the genetic markers in the genetic map of human beings consist of RFLPs andVNTRs. Because of their high degree of variability among people, VNTRs are also employed in DNA typing.The four haploid products of each of a series of meiotic divisions can be used to analyze linkage and recombinationin some species of fungi and unicellular algae. The method is called tetrad analysis.
In Neurospora and relatedfungi, the meiotic tetrads are contained in a tubular sac, or ascus, in a linear order, which makes it possible todetermine whether a pair of alleles segregated in the first or the second meiotic division. With such asci, it ispossible to use the centromere as a genetic marker; in fact, the centromere serves as a reference point to which allgenes in the same chromosome can be mapped. Linkage analysis in unordered tetrads is based on the frequenciesof parental ditype (PD), nonparental ditype (NPD), and tetratype (TT) tetrads.
The observation of NPD << PD is asensitive indicator of linkage.Recombination between alleles of the same gene demonstrated that genes are composed of a linear array ofsubunits, now known to be the nucleotides in the DNA. The functional definition of a gene is therefore not basedon mutation or on recombination but on complementation (by means of the complementation test). If two recessivemutations, a and b, are present in a cell or organism in the trans configuration and the phenotype is wildtype(complementation), the mutations are in different genes. If the phenotype is mutant (noncomplementation), themutations are in the same gene.
At the molecular level, lack of complementation implies that allelic mutationsimpair the function of the same protein molecule.Key TermsascosporeHaldane's mapping functionrecominationascusheterochromatinrepulsionattached-X chromosomeKosambi's mapping functionrestrictin endonucleasecentimorganlinkagerestriction fragment lengthpolymor-chromosome inteferencelinkage groupchromosome maplinkage mapsecond-division segregationchromatid interferencelocussimple tanderm repeatpolymorphismcoefficient of coincidencemap unitcompound-X chromosomemapping functionsometic mosaicscondensationnegative chromatid interferencetetratype (TT)couplingnonparental ditype (NPD)three-point crosseuchromainparental combinationstwin spotfirst-division segregationparental ditype (PD)variable number of tanderm repeatsfrequency of recombinationpositive chromatid interferencegenetic maprandom-spore analysisgenetic marketrecombinatphism (RFLP)(STRP)(VNTR)Page 164Chapter 4 GeNETics on the webGeNETics on the web will introduce you to some of the most important sites for finding genetic information onthe Internet.
To complete the exercises below, visit the Jones and Bartlett home page athttp://www.jbpub.com/geneticsSelect the link to Genetics: Principles and Analysis and then choose the link to GeNETics on the web. You willbe presented with a chapter-by-chapter list of highlighted keywords.GeNETics EXERCISESSelect the highlighted keyword in any of the exercises below, and you will be linked to a web site containing thegenetic information necessary to complete the exercise. Each exercise suggests a specific, written report thatmakes use of the information available at the site.
This report, or an alternative, may be assigned by yourinstructor1. The absence of crossing-over in males of Drosophila is a great convenience for detecting linkage. You can seethis for yourself at the keyword site. Mate females having a black body and dumpy wings with wildtype males.Then cross the F1 male progeny with the black, dumpy females. What are the phenotypes of the progeny and inwhat numbers do they occur? Does this result tell you anything about the linkage of black and dumpy? If assignedto do so, mate wildtype females with black, dumpy males. Make a diagram of the matings, indicating allgenotypes and phenotypes and the numbers observed in each generation of progeny.
Calculate the frequency ofrecombination between black and dumpy.2. The keyword Saccharomyces will connect you with the genome database of the yeast Saccharomycescerevisiae. Select Maps for graphical views of yeast chromosomes and Genetic Map to access the linkage map ofeach chromosome individually. If assigned to do so, use the site to find the relation between map length (incentimorgans) and physical length (in base pairs) of each of the yeast chromososomes. Make a bar graph of thisinformation.3. Detecting genetic linkage in human pedigrees requires special methods because human sibships are typicallyquite small.
For each(text box continued on next page)Review the Basics• What does it mean to say that two genes have a recombination frequency of 12 percent? How is the recombinationfrequency estimated?• For a region between two genes in which no more than one crossing over can take place in each cell undergoingmeiosis, the frequency of recombination is equal to one-half the frequency of meiotic cells in which a chiasmaoccurs in the region. Explain why the factor of one-half occurs when we compare recombination frequency tochiasma frequency.• What is a double crossover? How many different kinds of double crossovers are possible? Draw a diagram of eachkind of double crossover in a bivalent in a region between two genes, each heterozygous.• If each cell undergoing meiosis had two crossovers between the genes, and the types of double crossovers wereequally frequent, what would be the frequency of recombination between the genes?• What is interference? In a region of high interference, would you observe more or fewer double crossovers?• Explain why the recovery of ordered tetrads allows any gene to be mapped relative to the centromere of thechromosome on which the gene is located.Guide to Problem SolvingProblem 1: In Drosophila, the genes cn (cinnabar eyes) and bw (brown eyes) are located in the same chromosomebut so far apart that they appear unlinked.
Compare the possible offspring produced by the crossand the crossThe + symbols denote the wildtype alleles. Both cn and bw are recessive. Flies with the genotype cn cn havecinnabarPage 165(text box continued from previous page)genetic marker in a pedigree in which a trait, for example, a genetic disease, is present, the likelihood of linkage isassessed by a "lod score." The principal advantage of lod scores is that data from individual pedigrees can besummed.
Use the keyword to learn more about this approach and examine the sample lod score calculation for asimple pedigree. If assigned to do so, write a 250-word summary of the method and the logic on which it is based.MUTABLE SITE EXERCISESThe Mutable Site Exercise changes frequently.
Each new update includes a different exercise that makes use ofgenetics resources available on the World Wide Web. Select the Mutable Site for Chapter 4, and you will belinked to the current exercise that relates to the material presented in the chapter.PIC SITEThe Pic Site showcases some of the most visually appealing genetics sites on the World Wide Web. To visit theshowcase genetics site, select the Pic Site for Chapter 4.(bright red) eyes, bw bw flies have brown eyes, and the double mutant cn bw/cn bw has white eyes.Answer: The key to this problem is to remember that crossing-over does not take place in Drosophila males. Whenthe male is cn bw/++, his gametes are 1/2 cn bw and 1/2 + +, so the progeny are cn bw/cn bw (white eyes) and ++/cn bw (wildtype eyes) in equal proportions.
Crossing-over does happen in female Drosophila, but cn and bw areso far apart that they undergo independent assortment. When the female is cn bw/+ +, her gametes are 1/4 cn bw,1/4 cn +, 1/4 + bw, and 1/4 + + , so the progeny are cn bw/cn bw (white eyes), cn +/cn bw (bright red), + bw/cn bw(brown), and + +/cn bw (wildtype) in equal proportions.Problem 2: One of the earliest reported cases of linkage was in peas in the year 1905 between the gene for purpleversus red flowers and the gene for elongate versus round pollen.
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