MENDELIAN HEREDITY 287 



tion of a purple-white starchy-sweet cross shows purple starchy, 

 purple sweet, white starchy and white sweet grains. The sweet 

 grains are shrunken and so stand out in sharp contrast to the 

 smooth, plump starchy grains (Fig. 165). 



The pendulum has swung strongly toward the use of fruit flies 

 of the genus Drosophila in this work since the first important 

 contributions of Morgan and his associates. These insects have 

 several advantages for genetic research. They reproduce rapidly 

 and in large numbers; they are easily reared in the laboratory; 

 they are so organized as to facilitate cytological examination, and 

 best of all they give rise to many mutations which furnish the 



Fig. 165. — An ear of corn showing the F> dihybrid ratio. Four kinds of grains 

 are present, viz., purple starchy, purple sweet, white starchy and wnite 

 sweet, in the proportion of 282:80:74:27; this is very near to the expected 

 ration of 9:3:3:1. The starchy grains are smooth and the sweet grains 

 wrinkled. 



character contrasts so necessary for accurate observation (Fig. 

 155). Drosophila has been so productive that almost any principle 

 of genetics can be illustrated with it alone. 



Practical Importance. From the foregoing account it is appar- 

 ent that the transmission of characters, whatever their number, 

 goes on according to definite laws. By taking advantage of these 

 laws desired combinations of characters can be secured through 

 the proper control of propagation without awaiting the accidental 

 occurrence of the right individual. If a plant breeder has purple 

 sweet corn and white field corn, for example, and wishes to 

 produce white sweet corn, he can do so by hybridizing the two 

 and selecting the homozygous recessives of the Fo g(meration. 

 Or if any other characters arc desired the homozygous condition 

 will be found in some individuals of the F2 generation, and such 



