Chapter 10 



GENE ARRANGEMENT; 

 CROSSOVER MAPS 



I 



n the preceding chapter, the fre- 

 quency of crossing over was 

 presumed to be dependent upon 

 the distance between genes, the interval 

 being measured in crossover units. Differ- 

 ent genes linked to a given gene were found 

 to give different, essentially constant, cross- 

 over frequencies or crossover distances. Let 

 us now investigate how these different genes 

 are arranged spatially. 



Crossover distances can be used to study 

 whether linked genes are arranged in some 

 regular three-dimensional configuration such 

 as a sphere, cube, prism, or some two- 

 dimensional one such as a line, circle, or 

 triangle. To map the genes on the basis of 

 crossover data, that is. make a crossover {or 

 linkage) map, it is necessary to determine 

 all the crossover distances for a minimum 

 of three linked loci, since two points (such 

 as those defined by the crossover distance 

 between two genes) are not enough to de- 

 termine a specific geometrical arrangement. 



Gene Arrangement 



The arrangement of linked loci can be in- 

 vestigated with Drosophila. Using the three 

 X-linked genes, y (yellow body color), \v 

 (white eyes), and spl (split bristles), di- 

 hybrid females of the following types 

 are obtained: yw/-\ — (-; y spl -\ — |-; and 

 wspl/-\ — |-, and each type is test crossed 

 with the appropriate double recessive male. 

 The corresponding crossover distances are: 

 131 



y to w, 1.5; >• to spl, 3.0; and u> to spl, 1.5. 

 Since the crossover distance between y and 

 spl equals the sum of the crossover distances 

 from y to w and from w to spl, a linear ar- 

 rangement for these three genes is described, 

 namely, y w spl or spl w y. In other words, 

 the genetic map based on crossovers is linear. 

 If the reasonable assumption is made that 

 crossing over is a function of physical dis- 

 tance between genes, the genes are also 

 linearly arranged in the chromosomes. 



When the positions of a fourth X-linked 

 gene and all other X-linked genes are 

 mapped relative to the three studied above, 

 all are found to be arranged in a linear order 

 (Figure 10-1, and page s-16). In such a 

 crossover map, y is arbitrarily assigned the 

 position, or locus, zero. 



On a standard crossover map for the Dro- 

 sophila X, the genes y, w, spl, cv. ct, m, 

 and / line up respectively at positions 0, 1.5, 

 3.0, 13.7, 20, 36.1, 56.7, and one can see 

 that ct and spl are 17 map units apart 

 (20 — 3). Since one crossover map unit 

 equals one crossover per hundred gametes, 

 the dihybrid for spl and ct (Figure 10-2) 

 should produce 17% crossovers (8.5% 

 + + and 8.5% splct). However, such a 

 result is obtained only under special condi- 

 tions. 



The crossover frequency actually observed 

 will depend upon several factors. One of 

 these is the number of individuals making 

 up the sample. In small samples it is very 

 likely that, by chance, the observed values 

 will deviate considerably in both directions 

 from the standard map distance. As the 

 size of the sample increases, the observed 

 value will more closely approach the stand- 

 ard one. Standard distances, therefore, are 

 determined only after large numbers of prog- 

 eny have been scored. 



The relative viability (see p. 69) of dif- 

 ferent phenotypic classes is another factor 

 influencing observed crossover frequency. 



