MENDELISM 117 



resetted (#), long-haired (s), albino (p) guinea-pig, 

 all the offspring appear to be of one phenotypic 

 constitution, namely, resetted, short-haired, and pig- 

 mented (RSP). Their genotypic constitution is rep- 

 resented by the formula RrSsPp. These six factors 

 may form eight possible triple gametes, as follows : 

 RSP, RsP, RSp, Rsp, rSP, rSp, rsP, rsp. When two 

 germ-cells each made up of these eight triple gametes 

 unite in sexual reproduction, they will give rise to sixty- 

 four (8X8) possible zygotes as displayed in Figure 21. 



An analysis of Figure 21 shows among the offspring 

 eight different phenotypes in the ratio of 27 :9 :9 :9 :3 : 

 3:3:1 and 27 different genotypes in the proportions 

 indicated on page 116. The order of the three pairs 

 of symbols is changed from that in Figure 21 to 

 emphasize the fact that with independent unit char- 

 acters the order is immaterial. 



Sketches, drawn from photographs in Castle's "Ge- 

 netics and Eugenics," of the eight phenotypically differ- 

 ent guinea-pigs here described are shown in Figure 22. 



12. SUMMARY 



Three principles are concerned in Mendel's law: 

 / independent unit characters, dominance, and segrega- 

 tion. 



a. Independent Unit Characters. An organism, 

 although acting together as a physiological and mor- 

 phological whole, may be regarded from the point of 

 view of heredity as consisting of a large number of 

 independent heritable unit characters. 



