444 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



Haldane shows the calculated map distances (morgans) for all cross- 

 over percentages between 5 and 50. This table is based on the rela- 

 tions of the genes observed in the sex-linked group of Drosophila, but 

 it applies equally well to the second linkage group of Drosophila, and 

 to a group of three genes in the plant, Primula. Provisionally it may 

 be considered to be applicable generally to linkage systems in animals 



and in plants. 



TABLE IV 



A Table for Converting Cross-Over Percentages into Map Distances 

 ("Morgans") and Vice Versa (After Haldane) 



As an example of how the table may be used in predicting undeter- 

 mined linkage values, suppose that A is linked with B, and B with C 

 and that between A and B there are 10 per cent of crossovers. What 

 will be the cross-over percentage between A and C ? Converting the 

 observed cross-over percentages into map distances with the aid of the 

 table, we find the distance AB to be 10.3 and the distance BC to be 

 15.9. On the linear theory the distance AC will equal either the sum 

 or the difference of AB and BC, that is will be either 26.2 or 5.4. 

 Converting these distances into cross-over percentages by interpola- 

 tion in the table, we find that the cross-over percentage between A and 

 C should be either 23.7 or 5.1, according as the linear arrangement is 

 ABC or ACB. 



Measurement of linkage. — It will be observed that as the strength 

 of linkage increases, the cross-over percentage decreases. With a 

 cross-over percentage of 50, there is no linkage. With a cross-over 

 percentage of o, the linkage is complete, two characters so related 

 behaving as allelomorphs. Accordingly we depend upon the observed 

 cross-over percentage both for the detection of linkage and for the 

 measurement of its strength. But unfortunately the linkage strength 

 varies inversely as the cross-over percentage. This makes the cross- 

 over percentage directly considered, a rather poor measure of linkage 



