PLANT HYBRIDIZATION BEFORE MENDEL 297 



and laboriously detailed experiments leave no doubt as to the 

 central facts of what we know as the "Mendelian" factor-analysis, 

 viz., that each germ cell or gamete carries what Mendel called 

 the dominant or the recessive character as the case may be, in 

 pure form; that, in the hybrids, the gametes carry the dominant 

 and the recessive characters respectively in equal numbers, so 

 that when they unite at random according to the law of chance, 

 they will produce all possible combinations in equal numbers as 

 follows : 



Male gamete Tern ale gamete Zygote 



1. D X D DD 



2. D X R DR 



3. R X D RD 



4. R X R RR 

 This means that in any hybrid there exist equal numbers of 



these four combinations, when a single opposing pair of char- 

 acters is involved, so that the result of all the four possible com- 

 binations will be : 



25% 25% 25% 25% 



DD DR RD RR 



as the condition of things existing in any Mendelian monohybrid 

 (i.e., in which a single pair of characters only is considered), or, 

 as commonly expressed : 



25% 50% ■ 25% 



DD DR RR 



We have seen that, with an increase in the number of character- 

 pairs, we simply increase the number of terms in the series, by 

 the formation of a combination series, in which each kind of char- 

 acter-combination of the one unites with each kind of character- 

 combination of the other series — a process which can be repre- 

 sented by the algebraic multiplication of 



A + 2 Aa,+ a 

 by B + 2 Bb + b 



We can do this, because A-f-^Aa-j-a is, as we have seen, the 

 series of segregated types which the F^ or hybrid generation, Dr 

 (expressing the recessive by a small letter), algebraically ac- 

 tually can and does form on self-fertilization. Likewise, with the 

 series B+2Bb-f-b, into which the hybrid Bb segregates on self- 



