EXAMPLES OF MULTIPLE FACTORS. 19 



second generation. It is remarked that these occurrences strongly sug- 

 gest the results expected from a monohybrid Mendelian cross, although 

 the occurrence of every intergrade between the two parents would 

 not be expected. Shoemaker (1909) made crosses between cottons 

 having different types of leaves. After an intermediate Fi, he obtained 

 the three types in F 2 in the ratio of 1.27 :2.73 : 1. The intermediates 

 repeated this segregation in F 3 , while the parental types bred almost 

 true. This classification was made by inspection. From similar crosses 

 of leaves Balls (1909) concludes that the leaf form is undoubtedly a 

 compound character composed of several factors. In crosses involving 

 wide and narrow central lobes he found no definite grouping in F 2 , 

 using statistical methods. Forms wider and narrower than the parental 

 segments appeared, as well as intermediates. Some F 2 plants bred true. 



It would be possible to compare these results with those of Nilsson- 

 Ehle on the inheritance of the type of head in wheat. As stated above, 

 the length of head is supposed to be influenced by a dominant unit 

 (the compactum factor) whose presence is easily determined, and two 

 weaker ones which lack dominance. There may be present in one of 

 the cotton races one main unit without dominance, which influences 

 the leaf factor and which is absent in the other race. This would 

 account for the three modes in the second generation frequencies. 

 There may also be present one or more weaker units also influencing 

 the leaf factor, to whose segregation are due the sizes connecting the 

 three curves and those few intermediates appearing when the extreme 

 groups are inbred. 



A case that may be described as due to one strong unit and one or 

 more weaker ones is the inheritance of the size of plants in peas. 

 Mendel (1865) considered the bush habit (he called it "dwarf") due 

 to one dominant factor which, in crosses with long climbing plants, 

 gave a regular 3:1 ratio. When tall bush and short bush lines are 

 crossed an intermediate first generation results, followed in the next 

 generation by the appearance of tall and dwarf bush plants with all 

 intergrades. (Emerson, 1911, note 30, p. 88.) 



That the interaction of two factors is necessary to produce color is 

 a familiar conception. A color factor must be present before any factor 

 due to produce a certain color can become active. An albino, then, 

 may potentially have any color: it merely lacks the power to express 

 color. Baur (1907) has given clear evidence that the various red 

 colors of Antirrhinum are due to the action of certain factors, each of 

 which, in the presence of all lighter factors, produces a certain shade 

 of red. From this condition (which permits definite ratios) to one in 

 which the different shades of a color can not be easily separated is not 

 far. Tammes (1911) assumes a number of similar units that form the 

 blue in her Linum races (see p. 11). Different combinations of these 

 form a regular series of shades of blue. May it not be that the mono- 



