24 The Problems 



Similarly if the two gametes of two varieties distin- 

 guished by characters, A and B, which cannot be described 

 in terms of any common scale (such as for example the 

 "rose" and "single" combs of fowls) unite in fertilisation, 

 again the character of the mule form cannot be predicted. 

 Before the experiment is made the "mule" may present any 

 form. Its character or properties can as yet be no more 

 predicted than could those of the compounds of unknown 

 elements before the discovery of the periodic law. 



But again if the case be Mendelian the gametes borne 

 by AB will be either A's or J5's*, and the cross-bred 

 AB's breeding together will form A A's, Alt's and BB's. 

 Moreover, if as in the normal Mendelian case, AB's bear on 

 an average equal numbers of A gametes and B gametes, the 

 numerical ratio of these resulting zygotes to each other will be 



1 A A : 2AB : \ BB. 



We have seen that Mendel makes no prediction as to 

 the outward and visible characters of AB, but only as 

 to the essential constitution and statistical condition of its 

 gametes in regard to the characters A and B. Nevertheless 

 in a large number of cases the character of AB is known 

 to fall into one of three categories (omitting mosaics). 



(1) The cross-bred may almost always resemble one 

 of its pure parents so closely as to be practically 

 indistinguishable from that pure form, as in the 

 case of the yellow cotyledon-colour of certain varieties 

 of peas when crossed with green-cotyledoned varieties ; 

 in which case the parental character, yellow, thus 



* This conception was clearly formed by Naudin simultaneously 

 with Mendel, but it was not worked out by him and remained a mere 

 suggestion. In one place also Focke came very near to the same idea 

 (see Bibliography). 



