Alfred Sturtevant and the First Genetic

In 1909, Thomas Hunt Morgan taught the introduction to zoology class at Columbia University. Seated in the lecture hall were sophomore Alfred Henry Sturtevant and freshman Calvin Bridges. Sturtevant and Bridges were excited by Morgan's teaching style and intrigued by his interest in biological problems. They asked Morgan if they could work in his laboratory and, the following year, both young men were given desks in the "fly room," Morgan's research laboratory where the study of Drosophila genetics was in its infancy (see p. 000 in Chapter 4). Sturtevant, Bridges, and

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

Gene Mapping in Humans Mapping with Molecular Markers

Physical Chromosome Mapping

Deletion Mapping Somatic-Cell Hybridization In Situ Hybridization Mapping by DNA Sequencing

Morgan's other research students virtually lived in the laboratory, raising fruit flies, designing experiments, and discussing their results.

In the course of their research, Morgan and his students observed that some pairs of genes did not segregate randomly according to Mendel's principle of independent assortment but instead tended to be inherited together. Morgan suggested that possibly the genes were located on the same chromosome and thus traveled together during meiosis. He further proposed that closely linked genes —

Sturtevant's symbols: .

P ft

X chromosome locations: ■ !■ Modern symbols: y w / \ Yellow White body eyes

Vermilion eyes aaj m

Miniature wings

I 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.

Rudimentary wings those that are rarely shuffled by recombination—lie close together on the same chromosome, whereas loosely linked genes—those more frequently shuffled by recombination — lie farther apart.

One day in 1911, Sturtevant and Morgan were discussing independent assortment when, suddenly, Sturtevant had a flash of inspiration: variation in the strength of linkage indicated how genes were positioned along a chromosome, providing a way of mapping genes. Sturtevant went home and, neglecting his undergraduate homework, spent most of the night working out the first genetic map ( FIGURE 7.1). Sturtevant's first chromosome map was remarkably accurate, and it established the basic methodology used today for mapping genes.

Alfred Sturtevant went on to become a leading geneticist. His research included gene mapping and basic mechanisms of inheritance in Drosophila, cytology, embryology, and evolution. Sturtevant's career was deeply influenced by his early years in the fly room, where Morgan's unique personality and the close quarters combined to stimulate intellectual excitement and the free exchange of ideas. More details about Alfred Sturtevant's life

This chapter explores the inheritance of genes located on the same chromosome. These linked genes do not strictly obey Mendel's principle of independent assortment; rather, they tend to be inherited together. This tendency requires a new approach to understanding their inheritance and predicting the types of offspring produced. A critical piece of information necessary for predicting the results of these crosses is the arrangement of the genes on the chromosomes; thus, it will be necessary to think about the relation between genes and chromosomes. A key to understanding the inheritance of linked genes is to make the conceptual connection between the genotypes in a cross and the behavior of chromosomes during meiosis.

We will begin our exploration of linkage by first comparing the inheritance of two linked genes with the inheritance of two genes that assort independently. We will then examine how crossing over breaks up linked genes. This knowledge of linkage and recombination will be used for predicting the results of genetic crosses in which genes are linked and for mapping genes. The last section of the chapter focuses on physical methods of determining the chromosomal locations of genes.

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