LAW OF SEGREGATION 



623 



hybrids we have 50% recessives and 50% dominants in the next 

 generation. The production of plants or animals having dominant 

 and recessive character is based on the laws of chance. In hybrid 

 peas (Fi, page 622), for example, half the pollen grains would 

 bear germ cells con- 



yello* round 



Fi 



YY 



green wrinkledt 



RR 



Rr 



rR 



YY 

 Re 



yeUoVround 



Yy 





o 



ye\k*J round yellow round 



YY 

 Rr 



Yy 

 Rr 



yetloW round 



f 



Rr 



yellowfi?und yglb/f&und 



YY 

 rR 



/ellow^md 



Yy 



r "OJ 



yetiovvrutnd 



rP 



Yy 



rr 



O 



yeWcr^rcund gr<?«n round 



XT 



© 



XX 

 ri 



taining the deter- 

 miner of " smooth " 

 (R) and half the 

 determiner of " wrin- 

 kled " (r), while in 

 the ovule, half would 

 contain the deter- 

 miner " smooth " 

 and half " wrinkled." 

 Crossing them, we 

 have in the F2 gen- 

 eration, the result 

 shown in the dia- 

 gram. 



Dihybrids and 

 others. Though 

 breeding for one pair 

 of characters is com- 

 paratively easy to 

 understand, we often 

 find breeders cross- 

 ing for two or more 

 pairs of characters. 

 This is purely a 

 matter of mathematics (on paper) as the diagram on this page 

 shows, but it is too difficult to study in an elementary course in 

 biology. At the present time most of the really worth while 

 work in plant and animal breeding is being done by this method 

 of Mendel. Not only does this enable breeders to fix the unstable 

 hybrid characters, but also it enables them to combine worth- 

 while characters such as immunity to diseases of various kinds, 

 early ripening, color or size of fruit, and resistance to drought. 



yellovtMriflklal yelloWtfridWd ygap^'wriVOflgd gragrnfrinkled 



Dihybrids carry two characters. Here one parent is carry- 

 ing two dominant characters — yellow color and smooth seed 

 coat. The other parent is carrying two recessive characters — 

 green color and wrinkled seed coat. In Fi generation all four 

 characters are present but only the dominant ones show. If 

 these hybrids are crossed, the proportion oc the F2 generation 

 can be predicted if the characters are arranged as single pairs 

 abore and on the side of a series of squares. The possible 

 combinations of characters are gizen in the squares. 



