468 



AN INTRODUCTION TO ZOOLOGY 



letters Sg ; in 3 the letters Yw, and one only contains nothing but 

 wg. Since smoothness (S) and yellowness (Y) are dominants, the 

 result of the cross will be 9 plants bearing smooth yellow seeds, 

 3 bearing smooth green seeds, 3 bearing wrinkled yellow seeds, and 

 i bearing wrinkled green seeds. This result has been approximated 

 to quite closely in experimental results. 



vSY 



wY 



wg 



SY 



Sg 



wY wg 



Two other terms are used to indicate certain kinds of individuals. 

 The gametes or germ cells are always pure for certain factors, but 

 the zygote or individual is not. It may be pure with regard to a 

 certain character, say D, when we say it is a Homozygote. On the 

 other hand, it ma}^ be impure as in the F. I generation, in which case 

 we say it is a Heterozygote with regard to that particular pair of 

 allelomorphs. A Homozygote produces gametes all bearing the 

 same factor, a Heterozygote produces gametes bearing different 

 factors. Lastly, because of the way in which the recessive character 

 disappears in . the F. I generation in many of the crosses made and 

 then reappears again in the F. 2 generation, this particular type 

 of inheritance is called alternate, but as rioted above this pheno- 

 menon is by no means always manifested in crossing. 



The great value of Mendel's work and of its rediscovery was not 

 merely in the actual facts it brought to light, but in the stimulating 

 effect it had upon the experimental investigation of the problems 



