386 HYBRIDS, SEGREGATION 



vidual showing the recessive character ; of the former, however, 

 two will be heterozygotes, which would exhibit segregation 

 according to the same principle in the ensuing generation, whilst 

 the other, as well as the individuals possessing the recessive 

 character, are homozygotes. 



Further evidence, in support of this hypothesis, is furnished 

 by the result of crossing the hybrid of the first generation 

 with one or other parent (i.e. Aa x AA or Aa x aa). In 

 this case only two kinds of combinations will be obtained, 

 viz., taking the case in which the hybrid is crossed with the 

 parent-form having the dominant character, AA and Aa, and 

 there are obviously equal chances for either combination to 

 occur. The second generation of such a cross does, as a matter 

 of fact, afford individuals half of which are hybrid and half 

 pure. Moreover, when it is recalled that the endosperm of 

 Angiosperms develops as the result of a nuclear fusion (p. 368), 

 it is of interest to note that, if varieties of Maize possessing 

 different types of endosperm (i.e. variously coloured or con- 

 taining sugar and starch respectively) are crossed, that of the 

 resulting seeds exhibits evidence of its hybrid origin. 



If two pairs of characters are considered, it has been found 

 experimentally that, whilst the first generation consists only of 

 hybrids showing both dominants, the second comprises a number 

 of distinct individuals occurring in the following proportions : 

 9 with both dominant characters, 3 with one dominant and one 

 recessive, 3 with the other dominant and the other recessive, 

 and I showing both recessives. The relations wiU be plain if 

 the diagram in Fig. 225 is studied. Thus, if yellow round Peas 

 are crossed with green angular ones, the first generation all bear 

 yellow round Peas ; in the second generation there will be 

 9 yellow round, 3 yellow angular, 3 green round, I green 

 angular. Of the nine individuals showing both dominant char- 

 acters one only breeds true, as also does the one bearing both 

 recessive characters. The remainder, on being self-fertilised, 

 exhibit segregation according to the particular characters which 

 they contain. 



The dominance of a character is only important for the 

 elucidation of the observed facts in the many cases where 

 dominance occurs. But the principle of segregation is equally 



