THE GREAT RELAY RACE 



445 



progeny always fell into two groups in appearance like the two grand- 

 parents, in the ratio of 3:1. Thus, when smooth peas were arti- 

 ficially crossed with wrinkled peas, the hybrids were all smooth peas, 

 and when these smooth hybrids in turn were allowed to cross inter se, 



Parents CP) 



Gametes 



Hqbrid children (f,) 



Gametes 

 Grandchildre 



Diagram of the ancestry and progeny of a typical monohybrid, formed from 

 smooth and wrinkled garden peas. The inner circles represent germplasm, en- 

 closed in the outer circles, or somatoplasm. S, determiners of smooth peas; 

 s, determiners of wrinkled peas. 



the resulting grandchildren could be grouped in the ratio of three 

 smooth peas to one w^rinkled pea. These results are indicated dia- 

 grammatically in the accompanying figure, with the smooth charac- 

 teristic represented by a single letter as *S, and the wrinkled kind by 

 s. The germinal make-up of each individual is thus represented by 

 two letters, since it is always derived from two parental gametes. 



If smooth and wrinkled gametes come together in the same indi- 

 vidual, the smooth determiner covers up, or "dominates," the 

 wrinkled one, and is consequently called a dominant, while the 

 wrinkled gamete recedes from visible expression for the time being, 

 and is designated as a recessive. Which one of the alternate pair of 

 parental characters will be dominant and which recessive in the 

 offspring in any given case cannot be learned in advance by inspec- 

 tion. It is, therefore, necessary to resort to the breeding test in order 

 to make the determination. 



Further crosses on IVIendel's part showed that SS peas were pure 

 stock, like one of the grandparents with which he started, and when 

 interbred produced only SS peas, although coming from impure or 

 hybrid parents. Similarly the ss peas were also pure like the other 

 grandparent, and likewise always gave rise only to ss peas when 

 allowed to inbreed with their own kind. The hybrid Ss peas, on the 



