324 ON" HEREDITY IK EABBITS. 



A plaut is composed of unit characters whicla are represented 

 in tlie germ-cells, or gametes, by independent factors. 



Pure breeding involves the union of like factors : (A x A 

 = AA). 



Cross breeding involves the union of unliJce (but corresponding) 

 factors : (A X B = A B). 



Pure races give off'gametes carrying the one factor concerned : 

 (A A gives off Ag simply). 



Hybrid races give off two kinds of gametes, on the average in 

 equal numbers, each carrying one of the factors concerned in the 

 cross. No gamete carries hotJi (A B gives off As and Bs in equal 

 numbers, and no gamete carries A and B). 



This is known as gametic segregation. 



From this it follows that the offspring of the self-fertilized 

 hybrids A B are of three kinds : — 25 per cent. A A, 50 p. c. A B, 

 and 25 p. c. BB. 



The A As and B Bg, being gametically pure, breed true not- 

 withstanding their A B origin. 



This is known as gametic purity. 



The ABg, being hybrids, breed as such, gametic segregation 

 again taking place. 



"When more than one pair of unit characters are concerned in 

 the cross, the average result can be calculated on the above basis 

 by the simple calculus of chance. 



In his experiments Mendel encountered the phenomenon of 

 dominance, i. e. when certain pairs of unit characters were crossed, 

 one of the pair was always dominant over the other, which Mendel 

 termed recessive^ Ai x B = A (B). 



This dominance of A over B in the soma caused the offspring 

 of the self-fertilized hybrids to appear as 3 A : 1 B. Analysis in 

 the following generation, however, demonstrated their gametic 

 formula to be 1 A A : 2 A B : 1 B B, in accordance with the 

 Mendelian principles. C. C. H. 



28th February, 1905. 



