94 Heredity and Environment 



the proportion of (3:i) 3 — 27:9:9:9:3:3:3:1. Thus if a 

 pea with round (R) and yellow (Y) 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 x generation 

 have round and yellow seeds and tall stem, R, Y, and T being 

 dominant over IV, G, and D. In the F 2 generation there are 

 sixty-four possible combinations (genotypes) of these six char- 

 acters (Fig. 32) ; but since a recessive character does not develop 

 if its contrasting dominant character is present there are only 

 eight types which come to expression (phenotypes) and in the 

 following numbers : 27 RYT : 9 RYD : .9 RGT : 9 WYT'.i RGD: 

 3 WYD : 3 WGT: 1 WGD. Of these sixty-four genotypes 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 (d) ; inflated (E), or constricted 



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 inheritance formula of a plant having all seven of 

 the dominant characters is ABCDEFG ; of one having all of the 

 recessive characters abedefg. When two such plants are crossed 

 the inheritance formula of the hybrid is AaBbCcDdEeFfGg, and 

 since the dominant and recessive characters (or rather determin- 



