280 FOUNDATIONS OF BIOLOGY 



Mendelian ratio for two pairs of contrasting characters is 

 merely the monohybrid 3 : 1 expanded. Both rest on the 

 same fundamental assumption that there is an independent 

 assortment of the genes and that those for alternative char- 

 acters segregate both members of a pair of allelomorphs 

 can never occur in the same gamete. (Figs. 141, 142.) 



3. Trihybrids 



Similarly, Mendelian trihybrids, for example the cross 

 between tall Peas bearing yellow round seeds and dwarfs 

 bearing green wrinkled seeds, give in the F 2 generation 27 

 genotypes and 8 phenotypes; the relative number of indi- 

 viduals in each phenotype being in the proportion 27 : 9 : 9 : 

 9:3:3:3:1. Of course, in nature there are few instances 

 in which parents and offspring differ by only one, two, or three 

 characters, but since characters arising from each pair of 

 allelomorphs can usually be treated singly, expediency 

 demands that the analysis be made with respect to one 

 or two pairs at a time, which accordingly is the usual 

 method of procedure. (Fig. 143.) 



4. General Principles 



Before passing to certain modifications and extensions of 

 Mendelian principles, it may serve to clarify the subject if we 

 restate in slightly different form and then summarize the 

 essential facts thus far discussed on the basis of Mendel's 

 own work. 



Every cell of the soma of an individual bears a pair of 

 genes for each 'unit' character (e.g., size in the case of the 

 garden Pea), one member of each pair having been derived 

 from each gamete which contributed to the individual's make- 

 up. When both genes are identical (e.g., either SS or ss) they 

 are expressed in the soma (e.g., the plant is tall or dwarf). 

 The individual is homozygous with respect to size. But when 



