270 



AN INTRODUCTION TO MODERN GENETICS 



cated morphology of trabants, constrictions, etc., which allow of 

 detailed study.^ 



7. Gene Dijferences Between Species and Subspecies 



In all groups of related species which have been carefully investi- 

 gated, the different species have been found to exhibit the same gene 

 mutations. This would be expected if one of the main factors in species 



Fig. 121. Some Homologous Genes in Rodents. — Only allelomorphs of three 

 loci are shown. + indicates that the gene occurs in the normal type, W that it 

 is found in wild races, D that it is found in domesticated animals. 



(From Haidane.) 



formation has been chromosome rearrangement in the way discussed 

 in the last section. The most complete description of such parallelism 

 is that of Vavilov^ for the different crop plants ; even quite distantiy 

 related species of cereals show very similar mutant types. Among 

 animals, one of the most extensive series of data, other than that in 

 Drosophila on which the comparative cross-over maps are based, relates 

 to the coat colours of rodents.^ 



As might be expected, the same locus may be normally occupied by 

 different allelomorphs in different species, so that ordinary Mendelian 



^ Babcock and Navashin 1930, Hollingshead and Babcock 1930. 



2 Vavilov 1922. ^ Haidane 1927. 



