VARIATION AND HEREDITY 227 



of the individual, are always " pure " for one or the 

 other of such a pair of alternative characters, then the 

 dwarfs in the F 3 generation may arise from the fusion 

 of two gametes, both of which contain the " factor " 

 for dwarf ness. They breed true because the factor 

 for tall is absent. We are accustomed to speak of 

 such an individual as homozygous for such a character. 

 The same could be said of the tails that breed true. 

 All of the gametes 



produced by such T ^ D P 



a homozygous in- ' p 



dividual would be i 



of one and the 



T T(D) T(D) D F. 



same kind. On 



the other hand, if ' ' ^ ' ' ' ' ' I 



T TT(D)T(D) DTT(D) T(D)D D---F 3 



a gamete that car- | i 



ries the factor for T D F 4 



tallneSS fuses with FIG. 81. Diagram of the results of cross- 



.1 , . ing tall and dwarf peas; see text. (From 



one that carries p unne tt.) 

 the factor for 



dwarfness, the latter would become recessive and 

 the indiviual would be tall, but the gametes developed 

 by such an individual might be of the two kinds, 

 one carrying the factor for the tall, the other for 

 dwarf. An individual of this sort is called heterozy- 

 gous for such a character. Remembering that the 

 purity of the gamete with respect to these characters 

 is postulated, and that each kind is produced pre- 

 sumably in equal numbers, if fertilization occurs at 

 random, then by the law of chances there would 

 be twice the opportunity for heterozygous as for 



