56 HEREDITY 



D, and the absence of the necessary factor, or the 

 recessive condition, by R 



Mating. Offspring. 



DD DR RR 



Let us consider some of the implications of this theory 

 of inheritance. 



It implies, firstly, the purity of the reproductive cells 

 ^\'ith regard to the characters which they bear. A 

 particular reproductive cell may carry the factor for 

 tallness or dwarf ness, for " Chinese " petals or " star ' 

 petals, but the hybrid condition can not be represented 

 in a single reproductive cell. The hybrid condition can 

 be produced only by the union of male and female 

 reproductive cells which are dissimilar with regard to 

 the factor in question. 



Secondly, each male or female reproductive cell con- 

 tains a complete set of factors for determining the 

 characters of an individual. If we have a pure variety 

 of tall, round and green-seeded peas, each egg -cell and 

 each pollen-grain s^ill contain the factors for tallness, 

 and for roundness and greenness of the seed. The 

 result of a cross is therefore the same, whether this 

 or that variety be used as the male or as the female 

 parent. The common beliefs of breeders, that the male 

 is prepotent with regard to certain characters, the 

 female with regard to others, have either no real founda- 

 tion, or are due to the operation of causes which have 

 nothing to do ^vith heredity, in the strict sense. There 

 appear to be some interesting exceptions to this rule, 

 which we will consider later. 



Thirdly, the fertihsed egg, and therefore the 

 organism which develops from it, is a double structure. 



