LINKAGE 171 



tween (m + n) and (m + n - 2mn) . It will approach the 

 former for amounts of 5 or less, and the latter for amounts 

 of 45 or over. In a useful table Haldane shows the calculated 

 map distances (morgans) for all cross-over percentages be- 

 tween 5 and 50. This table is based on the relations of the 

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

 applies equally well to the second linkage group of Droso- 

 phila 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. 



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

 undetermined linkage values, suppose that A is linked with 

 B, and B with C and that between A and B there are 10 pvr 

 cent of crossovers, and between B and C, 15 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.''Z or 

 5.4. Converting these distances into cross-over percentiiges 

 by interpolation in the table, we find that the cross-over 



