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Linkage, Recombination, and 7 Eukaryotic Gene Mapping • Alfred Sturtevant and the First Genetic Map • Genes That Assort Independently and Those That Don’t • Linkage and Recombination Between Two Genes Notation for Crosses with Linkage Complete Linkage Compared with Independent Assortment Crossing Over with Linked Genes Calculation of Recombination Frequency Coupling and Repulsion The Physical Basis of Recombination Predicting the Outcome of Crosses with Linked Genes Testing for Independent Assortment Gene Mapping with Recombination Frequencies Constructing a Genetic Map with Two-Point Testcrosses • Linkage and Recombination Between Three Genes Gene Mapping with the Three-Point Testcross Alfred Henry Sturtevant, an early geneticist, developed the first genetic map. (Institute Archives, California Institute of Technology.) Gene Mapping in Humans Mapping with Molecular Markers • Physical Chromosome Mapping Deletion Mapping Somatic-Cell Hybridization In Situ Hybridization Alfred Sturtevant Mapping by DNA Sequencing and the First Genetic Map In 1909, Thomas Hunt Morgan taught the introduction to Morgan’s other research students virtually lived in the labo- zoology class at Columbia University. Seated in the lecture ratory, raising fruit flies, designing experiments, and dis- hall were sophomore Alfred Henry Sturtevant and freshman cussing their results. Calvin Bridges. Sturtevant and Bridges were excited In the course of their research, Morgan and his stu- by Morgan’s teaching style and intrigued by his interest in dents observed that some pairs of genes did not segregate biological problems. They asked Morgan if they could work randomly according to Mendel’s principle of independent in his laboratory and, the following year, both young men assortment but instead tended to be inherited together. were given desks in the “fly room,” Morgan’s research labo- Morgan suggested that possibly the genes were located on ratory where the study of Drosophila genetics was in its the same chromosome and thus traveled together during infancy (see p. 000 in Chapter 4). Sturtevant, Bridges, and meiosis. He further proposed that closely linked genes— 159 160 Chapter 7 Sturtevant’s symbols: X chromosome locations: Modern symbols: yw v m r Yellow White Vermilion Miniature Rudimentary body eyes eyes wings wings ◗ 7.1 Sturtevant’s map included five genes on the X chromosome of Drosophila. The genes are yellow body (y), white eyes (w), vermilion eyes (v), miniature wings (m), and rudimentary wings (r). Sturtevant’s original symbols for the genes are shown above the line; modern symbols are shown below with their current locations on the X chromosome. those that are rarely shuffled by recombination—lie close predicting the results of genetic crosses in which genes are together on the same chromosome, whereas loosely linked linked and for mapping genes. The last section of the chap- genes—those more frequently shuffled by recombina- ter focuses on physical methods of determining the chro- tion—lie farther apart. mosomal locations of genes. One day in 1911, Sturtevant and Morgan were dis- cussing independent assortment when, suddenly, Sturtevant Genes That Assort Independently had a flash of inspiration: variation in the strength of link- age indicated how genes were positioned along a chromo- and Those That Don’t some, providing a way of mapping genes. Sturtevant went Chapter 3 introduced Mendel’s principles of segregation home and, neglecting his undergraduate homework, spent and independent assortment. Let’s take a moment to review most of the night working out the first genetic map (F◗ IG- these two important concepts. The principle of segregation URE 7.1). Sturtevant’s first chromosome map was states that each diploid individual possesses two alleles that remarkably accurate, and it established the basic methodol- separate in meiosis, with one allele going into each gamete. ogy used today for mapping genes. The principle of independent assortment provides addi- Alfred Sturtevant went on to become a leading geneti- tional information about the process of segregation: it tells cist. His research included gene mapping and basic mecha- us that the two alleles separate independently of alleles at nisms of inheritance in Drosophila, cytology, embryology, other loci. and evolution. Sturtevant’s career was deeply influenced by The independent separation of alleles produces his early years in the fly room, where Morgan’s unique recombination, the sorting of alleles into new combinations. personality and the close quarters combined to stimulate Consider a cross between individuals homozygous for two intellectual excitement and the free exchange of ideas. different pairs of alleles: AABB ϫ aabb. The first parent, AABB, produces gametes with alleles AB, and the second www.whfreeman.com/pierce More details about Alfred parent, aabb, produces gametes with the alleles ab, resulting ◗ Sturtevant’s life in F1 progeny with genotype AaBb (FIGURE 7.2).Recombi- nation means that, when one of the F1 progeny reproduces, This chapter explores the inheritance of genes located on the combination of alleles in its gametes may differ from the the same chromosome. These linked genes do not strictly combinations in the gametes of its parents. In other words, obey Mendel’s principle of independent assortment; rather, the F1 may produce gametes with alleles Ab or aB in addi- they tend to be inherited together. This tendency requires a tion to gametes with AB or ab. new approach to understanding their inheritance and predict- Mendel derived his principles of segregation and inde- ing the types of offspring produced. A critical piece of pendent assortment by observing progeny of genetic information necessary for predicting the results of these crosses, but he had no idea of what biological processes pro- crosses is the arrangement of the genes on the chromo- duced these phenomena. In 1903, Walter Sutton proposed a somes; thus, it will be necessary to think about the relation biological basis for Mendel’s principles, called the chromo- between genes and chromosomes. A key to understanding some theory of heredity (Chapter 3). This theory holds that the inheritance of linked genes is to make the conceptual con- genes are found on chromosomes. Let’s restate Mendel’s two nection between the genotypes in a cross and the behavior of principles in terms of the chromosome theory of heredity. chromosomes during meiosis. The principle of segregation states that each diploid indi- We will begin our exploration of linkage by first com- vidual possesses two alleles for a trait, each of which is paring the inheritance of two linked genes with the inheri- located at the same position, or locus, on each of the two tance of two genes that assort independently. We will then homologous chromosomes. These chromosomes segregate examine how crossing over breaks up linked genes. This in meiosis, with each gamete receiving one homolog. The knowledge of linkage and recombination will be used for principle of independent assortment states that, in meiosis, Linkage, Recombination, and Eukaryotic Gene Mapping 161 P generation aa bb ן AA BB Gamete formation Gamete formation Gametes AB ab Fertilization F generation 1 Aa Bb Gamete formation Gametes AB ab Ab aB Original combinations of New combinations of alleles (nonrecombinant alleles (recombinant gametes) gametes) Conclusion: Through recombination, gametes contain new combinations of alleles. ◗ 7.2 Recombination is the sorting of alleles into new combinations. each pair of homologous chromosomes assorts indepen- dently of other homologous pairs. With this new perspec- tive, it is easy to see that the number of chromosomes in most organisms is limited and that there are certain to be more genes than chromosomes; so some genes must be present on the same chromosome and should not assort independently. Genes located close together on the same chromosome are called linked genes and belong to the same linkage ◗ 7.3 Nonindependent assortment of flower color group. As we’ve said, linked genes travel together during and pollen shape in sweet peas. meiosis, eventually arriving at the same destination (the same gamete), and are not expected to assort independently. How- ever, all of the characteristics examined by Mendel in peas did purple was dominant over red and long was dominant over display independent assortment and, after the rediscovery of round. When they intercrossed the F1, the resulting F2 prog- Mendel’s work, the first genetic characteristics studied in eny did not appear in the 9Ϻ3Ϻ3Ϻ1 ratio expected with inde- ◗ other organisms also seemed to assort independently. How pendent assortment (FIGURE 7.3). An excess of F2 plants could genes be carried on a limited number of chromosomes had purple flowers and long pollen or red flowers and and yet assort independently? round pollen (the parental phenotypes). Although Bateson, The apparent inconsistency between the principle of Saunders, and Punnett were unable to explain these results, independent assortment and the chromosome theory of we now know that the two loci that they examined lie close heredity soon disappeared, as biologists began finding together on the same chromosome and therefore do not genetic characteristics that did not assort independently. assort independently. One of the first cases was reported in sweet peas by William Bateson, Edith Rebecca Saunders, and Reginald C. Punnett Linkage and Recombination in 1905. They crossed a homozygous strain of peas having purple flowers and long pollen grains with a homozygous Between Two Genes strain having red flowers and round pollen grains. All the F1 Genes on the same chromosome are like passengers on a had purple flowers and long pollen grains, indicating that charter bus: they travel together and ultimately arrive at the 162 Chapter 7 Meiosis I Meiosis II Late Prophase Metaphase Anaphase Metaphase Anaphase Gametes Crossing over Recombinant chromosomes Genes may switch from one homologous chromosome In meiosis II, genes that ...will then assort ...and end up in to another by crossing over in meiosis I. are normally linked... independently... different gametes. ◗ 7.4 Crossing over takes place in meiosis and is responsible for recombination.

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