SUMMARY 889 



Summary. — 1. The number and kinds of gametes 

 produced by a hybrid depends on the number of differen- 

 tial genes involved in the cross and on whether any of 

 them are linked in the same chromosome-pairs. 



2. For any particular type of mating (i.e. back-cross 

 or interbreeding) the kinds and ratios of combinations 

 of genes (i.e., genotypes) which occur in the second 

 hybrid generation depends on the ratio and kinds of 

 gametes formed by the first generation hybrid. 



3. The ratio of phenotypes in the second hybrid genera- 

 tion depends on the way in which the individual genes 

 express themselves as well as on the ratio of gametes pro- 

 duced by the first generation hybrid. 



4. The comparatively simple ratios to be expected 

 according to the laws of Mendel (i.e., 1st and 2nd Laws) 

 are very often greatly modified by (1) linkage and cross- 

 ing-over, and (2) the way in which the genes express 

 themselves. » 



