PART I.-HISTORICAL. 

 THEORY OF MULTIPLE FACTORS. 



So much hybridization capable of a Mendelian interpretation has 

 been performed in the last ten years that there remains little doubt, 

 in the minds of investigators, that Mendel's Law is a correct descrip- 

 tion of the inheritance of certain types of characters. Of the three 

 striking parts of this law, dominance, second generation ratios, and 

 segregation, the last has come to be used almost synonymously with 

 Mendelism. Cases showing the lack or imperfection of dominance are 

 very common; a hybrid may even appear with a character entirely 

 different from any possessed by either parent; the simple Mendelian 

 ratio ([l+2] + l) n has been found to be variously modified; but the 

 seeming individuality, the apparently non-mixing nature of the sub- 

 stances upon which visible characters depend, stands firmly as the 

 basis of Mendelism. So, when segregation is found, it may be said that 

 the characters which show it are inherited in a Mendelian fashion. Seg- 

 regation need not mean the reappearance of the parental combinations 

 of characters in the second filial generation. Its primary theoretical 

 conception is that the units that entered the first hybrid generation 

 are uncontaminated in its germ cells, and as these units are independent 

 every combination of them may appear in the next generation. An 

 indication of segregation is the appearance in the second generation 

 of forms never found in the first generation. If the parents differed 

 by many characters it would be very unlikely that, in a small second 

 generation (F 2 ), any form genetically like either parent would result 

 from recombination, yet perfect segregation might occur. 



It would be possible to produce and cross two strains of garden peas 

 one of which was homozygous for all the seven dominant characters that 

 Mendel investigated, while the other was homozygous for the seven 

 corresponding recessive characters — that is, lacking all the dominant 

 characters. Such plants would be characterized as follows : (1) Seed, 

 smooth (A), yellow (B), with gray seed coat (C); pods, green (D), 

 inflated when dried (E); habit, tall (F); flowers, axial (G). (2) Seed, 

 wrinkled (a), green (b), with white seed coats (c); pod, white (d), when 

 dried leathery (e); habit, dwarf (/*), flowers, terminal (faciated) (g). 

 The gametic formulas would be ABCDEFG, and abcdefg. The first- 

 generation hybrids would have the appearance of the dominant parent ; 

 their zygotic formula would be AaBbCcT)dEeFfGg. Since each of these 

 seven pairs of units is separate and independent, every possible com- 

 bination of them will be found in the germ cells of this first generation, 

 namely 128 (following Mendel's formula 2 n , where n equals the number 



