180 Locating Genes on Chromosomes 



merely coincidence), which is the ratio between the observed 

 percentage of double crossovers and the number that would be 

 expected on the basis of chance. The coefficient of coincidence 

 in this problem would be 7.75/9.66 or 0.802. 



When the coefficient of coincidence is 1.00, there is no inter- 

 ference, and when it is 0, interference is 100 per cent. All values 

 between and 1 may be obtained ; and the higher the coincidence, 

 the smaller the amount of interference. 



Chromosome Maps Including More Than Three Loci 



If several genes are discovered in an organism, how can their 

 linkage relations and the order in which they would appear on a 

 chromosome be determined? For example, how could the genes 

 for black body (6), purple eyes (pr) , vestigial wings (vg) , curved 

 wings (c), plexus veins [px] , and speck (sp) , be mapped? These 

 genes are in the second chromosome of Drosophila melanogaster. 

 The characters are readily identified, and stocks of these genes 

 can be purchased for laboratory experiments. Without consid- 

 ering the actual sequence in which these genes were discovered 

 and in which their linkage relations worked out, and without 

 presenting the actual data, let us assume that in several experi- 

 ments consisting of three-point crosses, enough information has 

 been obtained to plot the genes. If a three-point cross involving 

 pr, vg, and px showed that the percentage of recombinations be- 

 tween pr and vg was 12.5, between vg and px was 33.5, and be- 

 tween pr and px was 46, the genes could be mapped in either 

 of the two orders: 



46.0 46.0 



Although one arrangement is the reverse of the other, the 

 sequence of the genes is the same in each case; and either ar- 

 rangement can be adopted arbitrarily with equal justification. 

 The first will be used. If the gene sp is now discovered, it could 

 be tested in a three-point cross with two of these other three 

 genes. If it were tested with vg and px and found to give 6.5 

 per cent of crossovers with px and 40.0 per cent with vg, the 



