Mendel's law of heredity and horticulture. 27 



" presence " of the tall factor is marked by the presence of black in the 

 spaces, while its " absence " is shown by the absence of black in the 

 spaces, leaving them apparently white. It will be observed that in the 

 pure tails there is a double presence of the tall factor made up of a 

 single presence from each parent, while in the impure tails there is only 

 a single presence from one parent. 



In other words, in the pure tails there is a double presence and m 

 the impure tails a single presence, while in the pure dwarfs there is a 

 complete absence of the tall factor. 



In the same way we may take it that the presence of the factor for 

 roundness prevents wrinkling and the pea is rounded, while in the 

 absence of the round factor the pea is wrinkled. Similarly the presence 

 of the factor for yellowness changes the green peas into yellow, while 

 in its absence the peas remain green, and so on with other characters. 



The Phenomenon of " Dominance." 



The " presence and absence " method puts a different construction 

 on the question of Mendelian " dominance. " From the standpoint of 

 " presence and absence " we cease to regard " dominance " as a 

 particular mode of inheritance. We look upon it simply as due to the 

 " presence " of a higher factor which somehow conceals the manifesta- 

 tion of a lower factor. Thus when we speak of a yellow pea as being 

 dominant " to a green pea we simply mean that the " presence " of 

 the higher " yellow " factor conceals the manifestation of the lower 

 " green " factor and the pea is yellow. Oti the other hand, in the 

 " absence " of the higher " yellow " factor, the lower green factor, 

 present all the time in the yellow pea, is manifested and the pea is 

 green. 



So far we have hardly any indications as to how the higher factors 

 conceal the " presence " of the lower factors, but it seems likely that 

 this phenomenon is part of the larger question of the interactions 

 between the various unit factors in the production of visible characters. 

 The complete solution of this important problem is for the future, and 

 when it is fully solved, no doubt, we shall know more about heredity 

 and variation. From the results of various experiments with plants 

 and animals we already know that complete " dominance " is far from 

 universal. In many cases the " dominance " is incomplete, the lower 

 " recessive " character being manifested in various degrees in the 

 cross-bred. 



In such cases it would appear that a single presence of a unit 

 factor produces a different result from a double presence of the same 

 factor. In other cases interaction between two unit-factors produces 

 a new form distinct from both, parents. It seems likely too, that the 

 familiar blended and mosaic forms met with, especially in hybrids 

 between distinct species, are also due to the varied interactions between 

 different unit-factors. All this goes to show that the phenomenon of 

 Mendelian " dominance," though important, is but a secondary con- 

 sideration quite distinct from Mendel's law of segregation. 



