Recombination

During meiosis, chromosomes come into close physical contact. This can lead to chromosome breakage and rejoining. In this process, an overlapping chromosome breaks at the point of contact with another chromosome, and the two ends of the broken chromosomes can swap positions. Usually, chromosomes break and join at the same position so there is no gain or loss of genes in this process. But the result is that genes present on different copies of the same chromosomes can

Figure 9.2 Inheritance of Two Pairs of Genes Located on the Same Chromosome. A. The genotype of double-homozygous parents are shown with both the eye color genes (R and r) and wing gene (B and b) on the same chromosome. B. Each parent produces only one type of gamete since they are homozygous. C. The offspring of the cross produces a double heterozygote, but note that the dominant form of the two genes are on the same chromosome and the recessive form of the two genes are also on the same chromosome. D. The double heterozygote offspring in this linked case produces only two types of gametes, not four types as seen in figure 9.1. E. There are only three different genotypes of offspring resulting from the double heterozygote parents if the genes are tightly linked on the chromosome.

Figure 9.2 Inheritance of Two Pairs of Genes Located on the Same Chromosome. A. The genotype of double-homozygous parents are shown with both the eye color genes (R and r) and wing gene (B and b) on the same chromosome. B. Each parent produces only one type of gamete since they are homozygous. C. The offspring of the cross produces a double heterozygote, but note that the dominant form of the two genes are on the same chromosome and the recessive form of the two genes are also on the same chromosome. D. The double heterozygote offspring in this linked case produces only two types of gametes, not four types as seen in figure 9.1. E. There are only three different genotypes of offspring resulting from the double heterozygote parents if the genes are tightly linked on the chromosome.

get shuffled. Let's take the above example of the mutant wings and black bodies of fruit flies. First, figure 9.2 shows the result in a situation where chromosomes do not break and rejoin. Here, the R-B and r-b genes stay together and only two phenotypic classes are observed: 75 percent of the flies have normal wings and normal body color, whereas 25 percent of the flies have mutant wings and black body color. No recombinants are seen. Morgan did not obtain these numbers because he did observe recombinants. Let us now imagine that these chromosomes, one with the R and B genes, and one with the r and b genes, break and rejoin between R and B and between r and b (figure 9.3). You can see that the R gene, originally linked to the B gene, is now associated with the b gene. Similarly, r is now associated with B. These new gene combinations are due to chromosome recombination and the resultant chromosomes are called recombinants. Individuals with the recombinant chromosomes are also called recombinants.

How often does recombination occur? It is the distance between genes along the chromosome that determines how often they recombine. You can imagine that breakage anywhere between the R/r and B/b genes will result in the new combination shown in figure 9.3. Thus the frequency of recombination depends roughly on the distance between R/r and B/b genes. For example, if the two genes are at the opposite ends of the chromosome, the chances of recombination are high. In that case, there will be many recombinants. If, on the contrary, the two genes are very close together, chances of recombination between the two genes are small. In Morgan's experiment, the distance between the two genes, R/r and B/b, was pretty close: he observed only 3 percent recombinants.

By determining the percentage recombination of linked genes in the offspring of a cross, we can map genes onto chromosomes. With fruit flies, if appropriate crosses are made, scientists can decide whether the genes are unlinked. This is the case when they observe a phenotypic ratio expected for independent assortment, for example a 9:3:3:1 ratio. If the ratio is significantly different from this expected ratio, they can conclude that the two genes are on the same chromosome, linked. Next, if two genes are linked, the proportion of individuals with phenotypes resulting from recombination is a measure of how far apart the genes are located on the chromosome. In fact, a standard unit of distances along chromosomes is the recombination recombinants

Figure 9.3 Breaking and Rejoining the Chromosomes Shown in Figure 9.2. The Punnett square shown in figure 9.2.E is expanded to include the chromosomes resulting from recombination. The original Punnett square is shown on the upper left enclosed by double lines. The recombinant chromosomes are added to the Punnett square. Those boxes marked by asterisk (*) represent the phenotypic classes not seen in the figure 9.2. Because the recombinant chromosomes represent only 6 percent of the chromosomes, the numbers of flies with the * phenotype are small.

Figure 9.3 Breaking and Rejoining the Chromosomes Shown in Figure 9.2. The Punnett square shown in figure 9.2.E is expanded to include the chromosomes resulting from recombination. The original Punnett square is shown on the upper left enclosed by double lines. The recombinant chromosomes are added to the Punnett square. Those boxes marked by asterisk (*) represent the phenotypic classes not seen in the figure 9.2. Because the recombinant chromosomes represent only 6 percent of the chromosomes, the numbers of flies with the * phenotype are small.

percentage, named centimorgans after T. H. Morgan. We can thus establish a chromosome map giving the relative positions of many genes, their distances measured as the amount of recombination observed between them.

We see that genes can be mapped on the chromosomes of Drosophi-la because we can do many crosses between flies with different genetic mutations. Then we can analyze hundreds of their offspring to see if the numbers correspond to ratios for independent assortment or not. In humans, we cannot do this. How do we measure linkage in humans?

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