HEEEDITY 



breed true, for it is homozygous, containing 

 only like units. The double recessive class, 

 long white, of course contains only homozygous 

 individuals, but in each class which shows a 

 dominant unit, heterozygous individuals out- 

 number homozygous ones, as 2 : 1 or 8 : 1. Now 

 the breeder who by means of crosses has pro- 



Short Dark. Lonj; Dark. Short White. Long White. 



ISD. SD iLD. I'D iS^^S^ iLW. Lw 



2SD. LD SLD. I'W es^i-w 



2S D. SW 

 ^ S D. I- ^ 

 IT "a a" 1 



Fia. 31. — Diagram showing the kinds and relative frequcncina 

 of the young to be expected in Fj from the crossing of 

 animals shown in Figs. 26 and 27. 



duced a new type of animal wishes, of course, 

 to " fix " it, — that is, to obtain it in a condi- 

 tion which will breed true. He must, therefore, 

 obtain homozygous individuals. If he is deal- 

 ing with a combination which contains only 

 recessive characters, this will be easy enough, 

 for such combinations are invariably homozy- 

 gous. His task will become increasingly diffi- 

 cult the more dominant characters there are 

 included in the combination which he desires 

 to fix. 



46 



