MENDELISM 195 



of P. sincnsis, P. pyramidalis, and P. stellata in the 

 ratio of i : a : I. 



Cases like the above illustrate the essential part of 

 Mendel's law even better than those in which domi- 

 nance is present, the characteristic proportion of one 

 of each homozygote type to two of the heterozygote 

 being at once recognisable in such a case without the 

 necessity for further breeding ; whereas, in cases where 

 there is dominance, further study is necessary in order 

 to distinguish, among the individuals of dominant 

 appearance, those which are pure dominant and those 

 which are heterozygous in constitution. 



In concluding our account of the simpler forms of 

 Mendelian phenomena we may consider one further 

 point with regard to the nature of the two allelomorphs 

 making up any particular pair. In what is probably 

 a majority of the cases hitherto examined the dominant 

 and recessive allelomorph seem to represent respec- 

 tively the presence and absence of something. Thus 

 the dominance of colour to absence of colour, or white- 

 ness, is a very frequent phenomenon. And in some of 

 the more complex cases to be described in the next 

 chapter we shall find the presence and absence of a 

 particular factor very often behaving as a pair of Men- 

 delian allelomorphs. The question arises as to how 

 far this conception should be extended. It seems, for 

 instance, somewhat far-fetched to speak of dwarf ness 

 as being simply determined by the absence of the factor 

 for tallness, though it is not impossible that this may 

 be the correct way of looking at the facts. Be this as 



it may, it is to be remembered that a Mendelian pair 



132 



