26 BREEDING CROP PLANTS 



and Pr. The lack of the factor may be represented by a small 

 letter. 



PARENTS APPEARANCE GAMETES Fi CROES 



Black Mexican Purple color PrRAC PrprRRAACc 



White Sweet White color prRAc 



As the FI seeds contain all factors necessary for the production 

 of purple color in the aleurone layer, they will be purple. In later 

 generations the factors R and A may be considered to be present in 

 each gamete, as both parents were homozygous for these characters. 

 The gametes of the F l plants will, therefore, be PrRAC, prRAC, 

 PrRAc, and prRAc. By the Punnet square method as illustrated 

 in the previous topic, the student may determine the possible F 2 

 combinations. These will be found to occur in the following 

 proportions : 



COMBINATIONS APPEARANCE 



1 PrPrRRAACC 



2 PrprRRAACC 



2 PrPrRRAACc 9 Purple aleurone 



4 PrprRRAACc 



1 prprRRAACC 



2 prprRRAACc 



1 PrPrRRAAcc 



2 PrprRRAAcc 

 1 prprRRAAcc 



3 Red aleurone 



4 White aleurone 



Linkage of Characters in Inheritance. Morgan and his co- 

 workers have made an intensive study of the inheritance of 

 characters in the fruit fly, Drosophila melanog aster. Over 300 

 inherited factors have been studied. If these inherited factors 

 are located in the chromosomes and as there are only four pairs 

 of chromosomes in the fruit fly, it would seem that certain fac- 

 tors should be linked together in their transmission. That this 

 is the case has been clearly proved by the result of many studies. 



What frequently happens is that factors which tend to be 

 present in the same chromosome or gamete, sometimes change 

 their linkage relations. That breaks do occur in the chromosomes 

 seems evident from careful cytological studies. Preparatory to 

 reduction division the two chromosomes which make up each 

 each pair lie side by side. There is often a twisting of these 

 chromosomes about each other and in some cases breaks occur 

 and the parts are joined in a new relation which causes a modi- 

 fication of the linkage relation. 



