Mechanism of Mendelian Heredity 235 



are ready for the production of a new individual, each 

 loses one set of homologous chromosomes in the so- 

 called reduction division, but the lost set is made up 

 indiscriminately of maternal as well as paternal chro- 

 mosomes, so that while one egg retains the maternal 

 chromosome A the other will retain the paternal one, 

 and so on. If before the reduction division all the eggs 

 had the chromosome constitution AAi^ BB^ CCx, DD^ 

 (where A B C D are the paternal and A^ B^ d D^ the 

 maternal chromosomes), after the reduction division 

 each daughter cell has a full set of four chromosomes, 

 but maternal and paternal mixed. Thus the one cell 

 may have AB^CD^, the other AxB^Cj^D, etc. This, 

 according to Sutton, is the basis of the Mendelian 

 heredity. Suppose the determiner of a certain char- 

 acter (violet colour of flower in the bean) is located 

 in a chromosome A of this species. The homologous 

 chromosome in beans with white colour may be desig- 

 nated as a. According to the chromosome theory of 

 Mendelian heredity a differs from A in one point, 

 though this difference is probably only of a chemical 

 character and not visible. 



If an egg with A is fertilized by a pollen with a 

 (or vice versa) , after fertilization the chromosome con- 

 stitution of the fertilized egg is Aa. All the other 

 homologous chromosomes are identical and therefore 

 need not be considered. All the nuclei of the Fj 

 generation have the chromosome constitution Aa, 



