734 A MANUAL OF VETERINARY PHYSIOLOGY 



When subsequently this zygote produces a generation, it will be found 

 that half its gametes carry the factor for tallness and half carry 

 that for shortness. This unblending or separation of the cells is 

 described as Segregation. A single gamete must, in this way, be pure 

 for either one or the other of two opposite characteristics ; in the 

 example selected it must be pure for tallness or pure for shortness, 

 inasmuch as we have seen that no blend of the two is possible. 



The whole Mendelian structure pivots on the segregation of 

 characters as outlined above, and the purity of the cells forming the 

 gamete. 



Had it been possible, we would have preferred to illustrate the 

 theory of Mendelism by reference to experiments on animals, but 

 Mendel's work was done on the common pea, and the essential facts 

 are known in this plant with the greatest precision. 



Selecting shortness and tallness from among several pairs of 

 contrasting characters which the pea furnishes, Mendel found, on 

 crossing a pure tall with a pure dwarf, that the offspring, or first 

 generation, were all tall. This settled that in the pea tallness is 

 dominant and shortness recessive. If the first generation be self- 

 fertilised, a second generation is obtained. These are no longer all 

 tall ; there are some short, and the proportion of tall to short plants 

 is as three to one. The dwarfs of this generation, if bred among them- 

 selves, never produce anything but dwarfs, and so on in every suc- 

 ceeding generation. ' The dwarfs, we have seen, are recessive, and 

 in Mendelian phraseology recessives breed true. The three tails 

 of the second generation, if bred among themselves, behave as 

 follows : One will produce all tall plants, and continue to do so to 

 infinity ; the remaining two tails behave differently. They do not 

 breed true, but each produces in a next generation tails and dwarfs, 

 in the proportion of three tails to one dwarf. Of these, one tall and 

 one dwarf breed true, while two tails are impure. These facts are 

 shown in the following table : 



Tall x Dwarf (Parents =P) 



All tall (First filial generation =Fi) 



Tall, Tall, Tall, Dwarf (=F 2 ), 



pure impure impure pure 



for for 



ever ever 



Reviewing the above classical example, it will be seen that the 

 first generation, though all tall, contained in their generative, as 

 apart from the body cells, the factors both for tallness and shortness. 

 There was nothing in the appearance of these tall plants to indicate 

 that they had a recessive character segregated, but it became evident 

 in the second generation that three different plants existed — e.g., a 

 true tall, a true short, and an impure tall. In the third generation 

 it was shown that the pure tall and pure short bred true, and that 

 the impure tall bred pure tails and dwarfs and impure tails, in the 

 proportion of three tails or dominants to one short or recessive. 



The above example is the simplest form of Mendelian inheritance, 

 and Mendel showed that each of the seven pairs of contrasting 

 characters found in the common pea all behaved in the same way — 



