Genes Can Be Identified by Their Map Position on the Chromosome

We will now consider a fundamentally different type of genetic analysis based on gene position. Studies designed to determine the position of a gene on a chromosome, often referred to as genetic mapping studies, can be used to identify the gene affected by a particular mutation or to determine whether two mutations are in the same gene.

In many organisms, genetic mapping studies rely on exchanges of genetic information that occur during meiosis. As shown in Figure 6-10a, genetic recombination takes place in germ cells after the chromosomes of each homologous pair have replicated, but before the first meiotic cell division. At this time, homologous DNA sequences on maternally and paternally derived chromatids can be exchanged with each other in a process known as crossing over. We now know that the resulting crossovers between homologous chromosomes provide structural links that are important for the proper segregation of pairs of homologous chromatids to opposite poles during the first meiotic cell division (for further discussion, see Chapter 19).

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FIGURE 6-10 Recombination during meiosis can be used to map the positions of genes. (a) Consider the gametes produced by an individual that carries two mutations, designated m1 (yellow) and m2 (green), that are on the maternal and paternal versions of the same chromosome, respectively. If crossing over occurs at an interval between m1 and m2 before the first meiotic division, then two types of recombinant gametes are produced; one carries both m1 and m2, whereas the other carries neither mutation. The longer the distance between two mutations on a chromatid, the more likely they are to be separated by recombination, and the greater the proportion of recombinant gametes produced. (b) In a typical mapping experiment, a strain that is heterozygous for two different genes is constructed. The frequency of parental or recombinant gametes produced by this strain can be determined from the phenotypes of the progeny in a testcross to a homozygous recessive strain. The genetic map distance in centimorgans (cM) is given as the percentage of the gametes that are recombinant.

Consider an individual with two different mutations, one inherited from each parent, that are located close to each other on the same chromosome. That individual can produce two different types of gametes according to whether a crossover occurs between the mutations during meiosis. If no crossover occurs between them, gametes known as parental types, which contain either one or the other mutation, will be produced. In contrast, if a crossover occurs between the two mutations, gametes known as recombinant types will be produced. In this example, recombinant chromosomes would contain either both mutations or neither of them. Recombination events occur more or less at random along the length of chromosomes; thus the closer together two genes are, the less likely that recombination will happen to occur between them during meiosis. Thus the less frequently recombination is observed to occur between two genes on the same chromosome, the closer together they must be. Two genes that are on the same chromosome and that are sufficiently close together that significantly fewer recombinant gametes than parental gametes are produced are considered to exhibit genetic linkage. If the number of recombinant gametes produced is not significantly less than the number of parental gametes, the two loci under consideration are considered to be unlinked and could be far apart on the same chromosome, or they could be on different chromosomes.

The technique of recombination mapping was devised in 1911 by A. Sturtevant while he was an undergraduate working in the laboratory of T. H. Morgan at Columbia University. Originally used in studies on Drosophila, this technique is still used today to assess the distance between two genetic loci on the same chromosome in many experimental organisms. A typical experiment designed to determine the map distance between two genetic positions involves two steps. In the first step, a strain is constructed that carries a different mutation at each of two positions, or loci. In the second step, the progeny of this strain are assessed to determine the relative frequency of inheritance of parental or recombinant types. A typical way to determine the frequency of recombination between two genes is to cross one of these heterozygous progeny with another individual that is homozygous for each gene. For such a cross, the proportion of recombinant progeny is readily determined because recombinant phenotypes will differ from the parental phenotypes. By convention, one genetic map unit is defined as the distance between two positions along a chromosome that results in 1 recombinant individual in 100 total progeny. The distance corresponding to this 1 percent recombination frequency is called a centimorgan (cM) in honor of Sturtevant’s mentor, Morgan (Figure 6-10b).

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A complete discussion of the methods of genetic mapping is beyond the scope of this introductory discussion; however, two features of recombination mapping need particular emphasis. First, the frequency of genetic exchange between two loci is proportional to the physical distance in base pairs separating them only for loci that are relatively close together (say, less than about 10 cM). For linked loci that are farther apart than this, a distance measured by the frequency of genetic exchange tends to underestimate the physical distance because of the possibility of two or more crossovers occurring within an interval.

A second important concept needed for interpreting genetic mapping experiments in different types of organisms is that although genetic distance is defined in the same way for different organisms, the relationship between recombination frequency (i.e., genetic map distance) and physical distance varies between organisms. For example, a 1 percent recombination frequency (i.e., a genetic distance of 1 cM) represents a physical distance of about 2.8 kb in yeast, compared with a distance of about 400 kb in Drosophila and about 780 kb in humans.

One of the chief uses of genetic mapping studies is to locate the gene that is affected by a mutation of interest. The presence of many different already mapped genetic traits, or genetic markers, distributed along the length of a chromosome permits the position of an unmapped mutation to be determined by assessing its segregation with respect to these marker genes during meiosis. Thus the more markers that are available, the more precisely a mutation can be mapped. In Section 6.4, we will see how the genes affected in inherited human diseases can be identified using such methods. A second general use of mapping experiments is to determine whether two different mutations are in the same gene. If two mutations are in the same gene, they will exhibit tight linkage in mapping experiments, but if they are in different genes, they will usually be unlinked or exhibit weak linkage.