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0262_0280_bi_c07_te 3/8/06 11:05 AM Page 279 11–5 Linkage and Gene Maps Section 11–5 1 FOCUS Objectives I f you thought carefully about Mendel’s principle of independent assortment as you analyzed meiosis, one question might have been bothering you. It’s easy to see how genes located on different chromosomes assort independently, but what about genes located on the same chromosome? Wouldn’t they generally be inherited together? Key Concept • What structures actually assort independently? Vocabulary 11.5.1 Identify the structures that actually assort independently. 11.5.2 Explain how gene maps are produced. gene map Gene Linkage The answer to these questions, as Thomas Hunt Morgan first realized in 1910, is yes. Morgan’s research on fruit flies led him to the principle of linkage. After identifying more than 50 Drosophila genes, Morgan discovered that many of them appeared to be “linked” together in ways that, at first glance, seemed to violate the principle of independent assortment. For example, a fly with reddish-orange eyes and miniature wings, like the one shown in Figure 11–18, was used in a series of crosses. The results showed that the genes for those traits were almost always inherited together and only rarely became separated from each other. Morgan and his associates observed so many genes that were inherited together that before long they could group all of the fly’s genes into four linkage groups. The linkage groups assorted independently, but all of the genes in one group were inherited together. Drosophila has four linkage groups. It also has four pairs of chromosomes, which led to two remarkable conclusions. First, each chromosome is actually a group of linked genes. Second, Mendel’s principle of independent assortment still holds true. It is the chromosomes, however, that assort independently, not individual genes. How did Mendel manage to miss gene linkage? By luck, or by design, six of the seven genes he studied are on different chromosomes. The two genes that are found on the same chromosome are so far apart that they also assort independently. Reading Strategy: Predicting Before you read, preview Figure 11–19. Predict how a diagram like this one can be used to determine how likely genes are to assort independently. As you read, note whether or not your prediction was correct. Vocabulary Preview Have student volunteers describe what a map is. Elicit the fact that maps show the locations of places and things. Ask: What do you think a gene map is? (It shows the locations of genes on a chromosome.) Reading Strategy 왔 Figure 11–18 The genes for this fruit fly’s reddish-orange eyes and miniature wings are almost always inherited together. The reason for this is that the genes are close together on a single chromosome. It is the chromosomes that assort independently, not individual genes. Gene Maps If two genes are found on the same chromosome, does this mean that they are linked forever? Not at all. Crossing-over during meiosis sometimes separates genes that had been on the same chromosome onto homologous chromosomes. Crossover events occasionally separate and exchange linked genes and produce new combinations of alleles. This is important because it helps to generate genetic diversity. As students read, encourage them to write down the main ideas that lead them to determine whether or not their prediction was correct. 2 INSTRUCT Gene Linkage Build Science Skills Using Models Students can construct a model of a chromosome with beads threaded on a pipe cleaner. The beads represent genes, and the pipe cleaner represents the chromosome. Challenge students to demonstrate why linked genes do not usually assort independently. Ask: Could there be exceptions to this? (Yes, if crossingover occurs.) How could crossingover affect the linked genes of a fruit fly? (Alleles could be exchanged between a maternal chromatid and a paternal chromatid.) Gene Maps SECTION RESOURCES Use Visuals • Teaching Resources, Lesson Plan 11–5, Adapted Section Summary 11–5, Section ve Summary 11–5, Worksheets 11–5,Sa Section e Review 11–5 • Reading and Study Workbook A, Section 11–5 • Adapted Reading and Study Workbook B, Section 11–5 • Biotechnology Manual, Lab 2 • iText, Section 11–5 • Transparencies Plus, Section 11–5 Tim Technology: r Print: Figure 11–19 As students study the gene map, ask: Would you expect more crossing-over events to occur between star eye and speck wing or between star eye and black body? Explain. (Star eye and speck wing; because these genes are located farther apart, it is more likely that a crossing-over event will occur between (continued) Introduction to Genetics 279 0262_0280_bi_c07_te 3/8/06 11:05 AM Page 280 Exact location on chromosome 11–5 (continued) them.) Explain that recombination rates are calculated by determining the percentage of recombinants produced in a cross. Recombinant offspring have a phenotype that is different from either parent. For example, in a cross between a homozygous male with a black body (bb) and vestigial wings (vv) and a heterozygous female (BbVv) with a brown body and normal wings, most of the F2 offspring will look like either parent. However some of the offspring, about 20%, will have either a black body and normal wings or a brown body and vestigial wings. 3 ASSESS Evaluate Understanding Chromosome 2 0.0 Aristaless (no bristles on antenna) 1.3 Star eye 13.0 Dumpy wing 31.0 Dachs (short legs) 0 10 20 30 48.5 Black body 51.0 Reduced bristles 54.5 Purple eye 55.0 Light eye 67.0 Vestigial (small) wing 75.5 Curved wing 99.2 Arc (bent wings) 104.5 Brown eye 107.0 Speck wing 왖 Figure 11–19 This gene map shows the location of a variety of genes on chromosome 2 of the fruit fly. The genes are named after the problems abnormal alleles cause, not the normal structure. Interpreting Graphics Where on the chromosome is the “purple eye” gene located? Draw a hypothetical gene map on the board. Have students tell which genes would have high frequencies of crossing-over and which would not. Reteach Have students diagram a crossingover event to show how genes that are located close together have a lower frequency of recombination than genes that are located far apart. 40 50 60 70 80 90 100 110 In 1911, a Columbia University student was working part time in Morgan’s lab. This student, Alfred Sturtevant, wondered if rates of crossing-over between genes in meiosis might be a clue to something important. Sturtevant reasoned that the farther apart two genes were, the more likely they were to be separated by a crossover in meiosis. That meant that he could use recombination frequencies to determine the distances between genes. Sturtevant gathered up several notebooks of lab data and took them back to his room. The next morning, he presented Morgan with a gene map showing the relative locations of each known gene on one of the Drosophila chromosomes, as shown in Figure 11–19. If two genes are close together, the recombination frequency between them should be low, since crossovers are rare. If they are far apart, recombination rates between them should be high. Sturtevant’s method has been used to construct genetic maps, including maps of the human genome, ever since. 11–5 Section Assessment Paragraphs should explain that if the genes are usually inherited together, they are located near each other on the same chromosome. If they were far apart, crossing-over events would make them appear to be located on different linkage groups. If your class subscribes to the iText, use it to review the Key Concepts in Section 11–5. Answer to . . . Figure 11–19 The “purple eye” gene is located at 54.5. 280 Chapter 11 1. Key Concept How does the principle of independent assortment apply to chromosomes? 2. What are gene maps, and how are they produced? 3. How does crossing-over make gene mapping possible? 4. Critical Thinking Inferring If two genes are on the same chromosome but usually assort independently, what does that tell you about how close together they are? Cause-Effect Paragraph In your own words, explain why the alleles for reddishorange eyes and miniature wings in Drosophila are usually inherited together. Include the idea of gene linkage. Hint: To organize your ideas, draw a causeeffect diagram that shows what happens to the two alleles during meiosis. 11–5 Section Assessment 1. It is the chromosomes that assort independently, not individual genes. 2. A gene map shows the relative locations of genes on a chromosome. The frequency of crossing-over between genes is used to produce a map of distances between genes. 3. The farther apart two genes are, the more likely they are to be separated during a crossover in meiosis. Therefore, the frequency of crossing-over is equal to the distance between two genes. 4. The two genes are located very far apart from each other.