© Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION Gene Linkage and © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC 4NOTGenetic FOR SALE OR DISTRIBUTIONMapping NOT FOR SALE OR DISTRIBUTION CHAPTER ORGANIZATION © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR4.1 SALELinked OR alleles DISTRIBUTION tend to stay 4.4NOT Polymorphic FOR SALE DNA ORsequences DISTRIBUTION are together in meiosis. 112 used in human genetic mapping. 128 The degree of linkage is measured by the Single-nucleotide polymorphisms (SNPs) frequency of recombination. 113 are abundant in the human genome. 129 The frequency of recombination is the same SNPs in restriction sites yield restriction for coupling and repulsion heterozygotes. 114 fragment length polymorphisms (RFLPs). 130 © Jones & Bartlett Learning,The frequency LLC of recombination differs © Jones & BartlettSimple-sequence Learning, repeats LLC (SSRs) often NOT FOR SALE OR DISTRIBUTIONfrom one gene pair to the next. NOT114 FOR SALEdiffer OR in copyDISTRIBUTION number. 131 Recombination does not occur in Gene dosage can differ owing to copy- Drosophila males. 115 number variation (CNV). 133 4.2 Recombination results from Copy-number variation has helped human populations adapt to a high-starch diet. 134 crossing-over between linked© Jones alleles. & Bartlett Learning,116 LLC 4.5 Tetrads contain© Jonesall & Bartlett Learning, LLC four products of meiosis. 135 PhysicalNOT distance FOR is SALE often—but OR not DISTRIBUTION NOT FOR SALE OR DISTRIBUTION always—correlated with map distance. 120 Unordered tetrads have no relation to the One crossover can undo the geometry of meiosis. 136 effects of another. 122 Tetratype tetrads demonstrate that crossing-over takes place at the four- 4.3 Double crossovers are revealed strand stage of meiosis and is reciprocal. 137 © Jones & Bartlettin three-point Learning, crosses. LLC 124 © TetradJones analysis & Bartlett affords a convenient Learning, LLC NOT FOR SALEInterference OR decreasesDISTRIBUTION the chance of NOTtest forFOR linkage. SALE OR DISTRIBUTION137 multiple crossing-over. 127 The geometry of meiosis is revealed in ordered tetrads. 139 Gene conversion suggests a molecular Varieties of maize. mechanism of recombination. 142 [© Katarzyna Citko/ © JonesShutterStock, & Bartlett Inc.] Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION TXX.XX A Head Goes Here 111 4TH PAGES © Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION. 4.6 Recombination is initiated by a Chapter Summary 145 double-stranded break in DNA. 143 Learning Outcomes 146 © Jones & BartlettRecombination Learning, tends LLC to take place at © IssuesJones & Ideas & Bartlett Learning, LLC 146 NOT FOR SALE ORpreferred DISTRIBUTION positions in the genome. 146 NOTSolutions: FOR Step SALE by Step OR DISTRIBUTION 146 THE HUMAN CONNECTION Concepts in Action: Problems for Solution 148 Starch Contrast 134 GENETICS on the web © Jones & Bartlett Learning, LLC © Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION tend to remain together in inheritance, a phenom- enetic mapping means determining the rela- enon known as linkage. Nevertheless, the linkage tive positions of genes along a chromosome. is incomplete. Some gametes are produced that have It is one of the main experimental tools in G© Jones & Bartlett Learning, LLC different ©combinations Jones & Bartlett of the white Learning, and miniature LLC genetics. This may seem odd in organisms in which the alleles than those in the parental chromosomes. The DNANOT sequence FOR SALEof the genome OR DISTRIBUTION has been determined. If NOT FOR SALE OR DISTRIBUTION new combinations are produced because homologous every gene in an organism is already sequenced, then chromosomes can exchange segments when they are what is the point of genetic mapping? The answer paired. This process (crossing-over) results in recom- is that a gene’s sequence does not always reveal its bination of alleles between the homologous chro- function, nor does a genomic DNA sequence reveal © Jones & Bartlett Learning, LLC © Jonesmosomes. & Bartlett The probabi Learning, lity of recombinationLLC between which genes interact in a complex biological process. NOT FOR SALE OR DISTRIBUTION NOT FORany two SALE genes OR serves DISTRIBUTION as a measure of genetic distance When a new mutant gene is discovered, the first between the genes and allows the construction of a step in genetic analysis is usually genetic mapping to genetic map, which is a diagram of a chromosome determine its position in the genome. It is at this point showing the relative positions of the genes. The lin- that the genomic sequence, if known, becomes useful, ear order of genes along a genetic map is consistent because in some cases the position of the mutant gene © Jones & Bartlett Learning, LLCwith the conclusion that ©each Jones gene &occupies Bartlett a well- Learning, LLC coincides with a gene whose sequence suggests a role NOT FOR SALE OR DISTRIBUTIONdefined position, or locusNOT, in the FOR chromosome, SALE OR with DISTRIBUTION in the biological process being investigated. For exam- the alleles of a gene in a heterozygote occupying ple, in the case of flower color, a new mutation may corresponding locations in the pair of homologous map to a region containing a gene whose sequence chromosomes. suggests that it encodes an enzyme in anthocyanin In discussing linked genes, it is necessary to distin- synthesis. But the function of a gene is not always © Jones & Bartlett Learning, LLC guish which© Jones alleles are& Bartlettpresent together Learning, in the LLCparen- revealed by its DNA sequence, and so in some cases, NOT FOR SALE OR DISTRIBUTION tal chromosomes.NOT FOR This SALE is done OR by DISTRIBUTION means of a slash further genetic or molecular analysis is necessary to (“/”). The alleles in one chromosome are depicted sort out which one of the genes in a sequenced region to the left of the slash, and those in the homologous corresponds to a mutant gene mapped to that region. chromosome are depicted to the right of the slash. In human genetics, genetic mapping is important For example, in the cross AA BB 3 aa bb, the geno- because it enables genes associated with hereditary © Jones & Bartlett Learning, LLC © Jonestype & of Bartlett the doubly Learning, heterozygous LLC progeny is denoted diseases, such as those that predispose to breast can- A B/a b because the A and B alleles were inherited NOT FOR SALEcer, to ORbe localized DISTRIBUTION and correlated with the genomicNOT FOR SALE OR DISTRIBUTION in one parental chromosome and the alleles a and b sequence in the region. were inherited in the other parental chromosome. In this genotype the A and B alleles are said to be in the 4.1 Linked alleles tend to stay coupling or cis configuration; likewise, the a and together© inJones meiosis. & Bartlett Learning, LLCb alleles are in coupling.© Among Jones the & Bartlettfour possible Learning, LLC NOT FOR SALE OR DISTRIBUTIONtypes of gametes, the A NOTB and FOR a b types SALE are OR called DISTRIBUTION In meiosis, homologous chromosomes form pairs in parental combinations because the alleles are in the prophase I by undergoing synapsis, and the individ- same configuration as in the parental chromosomes, ual members of each pair separate from one another and the A b and a B types are called recombinants at anaphase I. Genes that are close enough together ( FIGURE 4.1, part A). in© the Jones same chromosome & Bartlett might Learning, therefore LLC be expected Another© Jones possible & configuration Bartlett Learning, of the A, a andLLC B, b toNOT be transmitted FOR SALE together. OR DISTRIBUTION Thomas Hunt Morgan allele pairsNOT is A FOR b/a B .SALE In this caseOR theDISTRIBUTION A and B alleles examined this issue using two genes present in the are said to be in the repulsion or trans configuration. X chromosome of Drosophila. One was a mutation Now the parental and recombinant gametic types are for white eyes, the other a mutation for miniature reversed (Figure 4.1, part B). The A b and a B types are wings. Morgan found that the white and miniature the parental combinations, and the A B and a b types © Jones & Bartlettalleles present Learning, in each XLLC chromosome of a female do© Jonesare the& Bartlett recombinants. Learning, LLC NOT FOR SALE OR DISTRIBUTION NOT FOR SALE OR DISTRIBUTION 112 CHAPTER 4 Gene Linkage and Genetic Mapping 4TH PAGES © Jones & Bartlett Learning, LLC. NOT FOR SALE OR DISTRIBUTION. (A)–Parental alleles in coupling (B)–Parental alleles in repulsion The degree of linkage is (A)–or cis configuration (B)–or trans configuration measured by the frequency © Jones & Bartlett Learning, LLC © Jones & BartlettA Learning,B LLC A b of recombination. NOT FOR SALE ORa DISTRIBUTIONb a NOTB FOR SALE OR DISTRIBUTION In his early experiments with Drosophila, Morgan found mutations in each of several Meiosis Meiosis X-linked genes that provided ideal materi- als for studying linkage. One of these genes, © Jones & Bartlett Learning, LLC