Phenomena of Inheritance 95 



dividuals are heterozygous and show Mendelian splitting in the 

 next generation. 



Trihybrids. When parents differ in three contrasting charac- 

 ters there are twenty-seven genotypes (3 3 ) and eight phenotypes 

 (2 3 ) in the F 2 generation in the proportion of (3: i) 3 = 27:9: 

 9 : 9 : 3 : 3 : 3 : r - Thus if a pea with round (R) and yellow ( F) 

 seeds and with tall (T) stem is crossed with one having wrinkled 

 (IV) and green (G) seeds and dwarf (D) stem all the progeny 

 of the F! generation have round and yellow seeds and tall stem, 

 R, Y, and T being dominant over W, G, and D. In the F 2 gener- 

 ation there are 64 possible combinations (27 genotypes) of these 

 six characters (Fig. 32) ; but since a recessive character does not 

 develop if its contrasting dominant character is present there are 

 only eight phenotypes which come to expression and in the 

 following ratios: 27 RYT:g RYD:g RGT:g WYT:$ RGD: 

 3 IVYD : 3 WGT: i WGD. Of these sixty- four combinations only 

 eight are homozygous and breed true (those lying in the diagonal 

 between upper left and lower right corners in Fig. 32), while only 

 one is a pure dominant and one a pure recessive (the ones in the 

 upper left and lower right corners of Fig. 32). 



3. Inheritance Formulae. Mendel represented the hereditary 

 constitution of the plants used in his experiments by letters em- 

 ployed as symbols, dominant characters being represented by capi- 

 tals and recessives by small letters. The seven contrasting char- 

 acters of his peas could be represented as follows : 



Seeds, round (A), or wrinkled (a) ; yellow (B), or green (b) ; 

 with gray seed coats (C), or white seed coats (c). 



Pods, green (D), or yellow (rf) ; inflated (E), or constricted 



(0- 



Habit, tall (F), or dwarf (/). 



Flowers, axial (G), or terminal (g). 



It is possible for one plant to have all of these dominant char- 

 acters or all of the recessive ones, or part of one kind and part of 

 the other. The gametic formula of a plant having all seven of 



