Mechanism of ]\Iendclian Heredity 231 



for pigment should be dominant over the absence of 

 such a factor, which is usually the case, inasmuch as 

 the cross of a coloured rat or rabbit with an albino is 

 black or coloured. There is, however, also a case where 

 whiteness is dominant over colour, as we shall see later. 

 This fact does not necessarily contradict the presence 

 and absence theory.^ 



When two pure breeds of parents differ in one char- 

 acter, e. g.y two varieties of beans, one with a violet the 

 other with a white flower, the cross between the two 

 species (the Fi generation) has pale violet flowers, 

 approximately intermediate between the two parents. 

 If these hybrids are bred among themselves the off- 

 spring is called the F2 generation. According to 

 Mendel's law the hybrids of the first Fj generation all 

 have two kinds of eggs in equal numbers, one kind 

 representing the pure breed of the parents with violet, 

 the other of the pure breed with white flowers. The 

 same is true for the pollen cells. Hence the following 

 possible combinations must appear in the offspring 

 when the pale violet hybrids are inbred: 



violet white eggs 



violet white pollen 



The four possible combinations are: (i) violet — violet; 



* The reader will find a critical discussion of the presence and absence 

 theory on page 220 of Morgan, Sturtevant, Mullcr, and Bridges, The 

 Mechanism of Mendelian Heredity. New York, 19 15. 



