Duplicate genes for capsule-form in Bursa bursa-pastoris. 129 



the two parents in respect to the particular character under consider- 

 ation, on the assumption (in some cases specifically stated) that such 

 a character is not affected by heterosis. The intermediacy of a char- 

 acter in the Fi does not prove, however, that heterosis has no effect 

 upon this particular character, especially when the parent stocks are 

 complex hybrid material like maize. In such material, the size any 

 organ whould have if all the determiners possessed by the given indi- 

 vidual were homozygous, is unknown, and consequently there is no 

 secure basis on which to predict the purely genetic (unstimulated) con- 

 dition of such a character in the F t , from its stimulated condition in 

 the parents. For example, because the length of ears in the Fi from 

 a cross between Tom Thumb pop-corn and Black Mexican sweet-corn 

 is intermediate between the ear -lengths of the parents, EAST and 

 HAYES (1911 p. 124) say that "ear-length does not show the increased 

 vigor due to heterozygosis that is seen in the heights of plants." The 

 two parents were in this case grown from commercial seed and were 

 both undoubtedly in a complex condition. These authors may have 

 intended to say merely, that the effect of heterosis is not as obvious 

 in ear-lengths as in height of stems, for EMEESON and EAST (1913, p. 40) 

 in discussing the same cross state that ear-length in maize "is not 

 affected by heterozygosis to as great a degree as height, although 

 some effect may be traced" *). In my crosses among nearly homo- 

 zygous strains of maize, there were always a great increase in ear- 

 length in the FI, and decreasing ear-length in subsequent generations, 

 showing that, at least in some cases, ear-length is markedly subject to 

 increase by heterosis. 



An increased F2 variability following an intermediate average 

 value in Fi might result from the bringing together of as few as two 

 Mendelian genes affecting different elements in the given character, 

 the one acting in a positive direction, the other in a negative direction, 

 and the dominance or lack of dominance would be immaterial, so long 

 as the effects of the genes in question were slight compared with the 

 fluctuations of the same character. Thus, if a plant possessing a 

 partial inhibitor or reducer of internode-number be crossed with another 

 plant having a stimulator for internode-length, all the other genes being 

 the same in the two cases, the height of the Fi plants would be inter- 

 mediate between the heights of the parents, with variability due alone 

 to fluctuation, as it is in the homozygous parents. The F would show 



1 ) Italics are mine. 



