172 MENDELISM 



formula. It indicates the proportion in which the 

 two pure types and their hybrid brethren will appear, 

 on breeding together the offspring of a simple or 

 mono-hybrid cross i.e., one in which attention is 

 paid to the behaviour of a single pair of characters only. 



So far we have been dealing with a pair of characters 

 consisting in the presence and absence respectively of 

 a particular pigment. Precisely similar results are to 

 be obtained in the case of a pair of structural char- 

 acters. The endosperm, or reserve substance, of cer- 

 tain varieties of Indian corn shows a smooth surface, 

 and contains an essentially starchy reserve material, 

 whilst in other races the surface of the endosperm is 

 wrinkled and the reserve product is of a sugary nature. 

 This sugary endosperm is characteristic of the kinds 

 of corn largely used in the United States of America 

 as a table vegetable. 



On crossing together a variety with smooth starchy 

 grains and one with wrinkled sugary grains, the grains 

 immediately resulting are smooth and starchy, no 

 matter whether the starchy strain is used as the seed- 

 parent or as the pollen-parent that is to say, the starchy 

 character is dominant, a dominant character being one 

 which appears in F a to the complete or almost com- 

 plete exclusion of the corresponding character ex- 

 hibited by the other parent, which is spoken of as 

 recessive. In the present case the sugary character is 

 recessive. 



The further behaviour of the cross between smooth 

 and wrinkled is precisely the same as that of yellow 



