58 HEREDITY [ch. 



taining- a double set of determinants for the various 

 characters of the plant, one determinant of each 

 pair being derived from the male parent, the other 

 from the female. An individual produced by union 

 of two germ-cells (gametes) and having this double 

 character is called a ' zy gote / The F^ zygote thus 

 contains a determinant for tallness derived from one 

 parent, and a corresponding determinant in which 

 the tall factor is absent derived from the second 

 parent. Now Mendel's hypothesis to account for the 

 observed facts was that although the zygote produced 

 by union of tall-bearing and short-bearing gametes 

 contains both factors, yet when this hybrid zygote 

 gives rise to gametes, it produces some bearing 

 tallness and others bearing shortness, but none 

 bearing both determinants ; i.e. that the alternative 

 characters segregate from each other in the forma- 

 tion of the gametes, and that gametes bearing one 

 or other of the two conditions are formed in equal 

 numbers. Since large numbers of gametes of each 

 kind are formed, and since they meet indiscriminately 

 in fertilisation, a tall will equally often meet a tall or 

 a short, and a short will equally often meet a tall and 

 a short, and the combinations will thus be in the ratio 

 of \TT, \Tt, UT, Ut, or ITT, 2Tt, \U. If this 

 hypothesis is true, it can be tested by fertilising the 

 Fi hybrid zygote with the pure parental types ; the 

 Fi zygote produces equal numbers of T and t gametes, 



