III. OBSERVATIONS ON THE OCCURRENCE OF LINKAGE 



IN RATS AND MICE. 



In publication 241 of the Carnegie Institution evidence was pre- 

 sented showing that the red-eyed yellow and pink-eyed yellow varia- 

 tions of the common rat (Mus norvegicus) are due to genes which are 

 linked with each other. Upon crossing with each other the two yellow 

 variations, which visibly differ in eye-color only, young are obtained 

 which differ from both parent races in coat-color as well as in eye- 

 color. These young are black-coated or gray-coated and have black 

 eyes. This result shows clearly that the two variations, which are 

 both recessive in genetic behavior, are due to independent genes. 



In the F 2 generation the two yellow varieties were recovered, each 

 with its distinctive eye-color, and certain individuals, which visibly 

 were pink-eyed yellows, were found from breeding tests to carry the 

 genes for red-eyed as well as for pink-eyed yellow. These double 

 recessives obviously had arisen by the process known as "crossing- 

 over," in which genes, although introduced in a cross by different 

 parents, yet later emerge together in the same gamete formed by an 

 F! individual. It is supposed that genes which behave in this way lie 

 in homologous chromosomes and that when crossing-over occurs a 

 gene (A) leaves the chromosome in which it originally lay and crosses 

 over into the homologous chromosome in which the other gene (B) 

 lay. Thus both A and B come to lie in the same chromosome and 

 at gametogenesis pass into the same gamete. From an examina- 

 tion of the proportion of the double recessive yellow individuals 

 found among the F 2 yellows, it was concluded that cross-over gametes 

 (those which carry genes for both yellow variations or for neither) 

 represent about 18.5 per cent of all the gametes formed by FI individ- 

 uals. If no linkage occurred, such gametes would form 50 per cent of 

 the total. 



To test more fully the strength of the linkage between these two 

 genes and to find out whether this linkage has the same strength in 

 both sexes, further experiments have been undertaken. A race of 

 homozygous double recessives (genetically both pink-eyed and red- 

 eyed) was built up from the F 2 cross-over individuals and with this 

 race FI individuals were crossed. If we designate by r the gene for 

 red-eyed yellow and by p the gene for pink-eyed yellow, an FI individual 

 might be expected to form gametes of the four sorts PR, Pr, pR and 

 pr. Of these 4 combinations, Pr and pR would correspond with 

 those furnished by the parent races, red-eyed yellow and pink-eyed 

 yellow respectively; but the other two, PR and pr, would be new and 



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