Chapter *17 



GENE ARRANGEMENT 

 AND CHIASMATA 



I 



't was found in the preceding 

 Chapter that the frequency of 



.crossing over between genes may 

 be considered in terms of distance measured 

 in map units. Different genes Hnked to a 

 given gene were found to have different cross- 

 over distances from it. The question arises, 

 how are these different genes related to each 

 other spatially? It may be that the crossover 

 distances between a number of linked genes 

 represent physical distances from one gene 

 to another which would describe either some 

 three-dimensional configuration (like a sphere, 

 cube, prism, or polyhedron) or some two- 

 dimensional one (like a circle, ellipse, tri- 

 angle, or line). In order to determine if 

 this is so, it is necessary to determine all the 

 map distances for a minimum of three linked 

 loci. (The crossover distance between two 

 genes defines only two points; two points are 

 insufficient to tell us whether linked genes 

 occur in a specific geometrical arrangement.) 

 The arrangement of linked loci can be in- 

 vestigated in Drosophila using the three X- 

 linked genes, y (for yellow body color), iv (for 

 white eyes), and spl (for split bristles). Dihy- 

 brid females of each of the following types are 

 obtained, y w/-\- -f , y spl/-{- +, w spl/-\- +, 

 and each is backcrossed to the appropriate 

 double recessive male. The crossover per- 

 centages, or map distances, obtained from 

 these crosses are, respectively, y io w \.5,y to 

 spl 3.0, and w to spl 1 .5. These data describe 

 a simple arrangement for these three genes, 

 namely a linear one, since the crossover dis- 



128 



tance between the end genes j and spl equals 

 the sum of the crossover distances from the 

 middle gene w to the end ones. We shall 

 presume, henceforth, that crossover distance 

 is proportional to physical hnear distance 

 between genes. 



It is also possible to study a fourth and 

 then all other X-linked genes, and to map 

 their positions relative to these three. When 

 this is done, it is found that all are arranged in 

 a linear order. In such a crossover or genetic 

 map, y is arbitrarily assigned the position, or 

 locus, 0. A standard crossover map for the 

 Drosophila X would have the genes y, w, spl, 

 cv, ct, m, and / lined up at the respective 

 positions, 0, 1.5, 3.0, 13, 21, 36, 56.7. From 

 this standard map one can read that ct and m 

 are 15 map units apart (36-21). Since one 

 map unit equals one crossover per hundred 

 gametes, the dihybrid for ct and m (Fig- 

 ure 17-1) should produce 15% crossovers 

 (7.5% + + and 7.5%, ct m). However, such 

 a result is obtained only when certain condi- 

 tions are met. 



The crossover rates actually observed will 

 depend upon several factors. One of these 

 is the number of individuals making up the 

 sample. Standard distances are arrived at 

 only after scoring large numbers of progeny. 

 In small samples it is very likely that, by 

 chance, the observed values will deviate con- 

 siderably from the standard map distance in 

 both directions. The larger the size of the 

 sample, the closer will the observed value 

 approach the standard one. 



Another factor influencing observed cross- 

 over rates concerns the fact that different 

 phenotypic classes may have different viabili- 

 ties (cf. p. 9). Usually the phenotypic ex- 

 pression of a + allele is more viable than that 

 of its mutant forms. So, in Figure 17-1, for 

 example, phenotypically ct m sons are not 

 as viable as normal (wild-type) sons, and 

 though both types are equally frequent as 

 zygotes, the former fail to complete develop- 

 ment more often than the latter, and are, 



