Page:Mendel's principles of heredity; a defence.pdf/44

 Similarly if the two gametes of two varieties distinguished 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 As or Bs, and the cross-bred ABs breeding together will form AAs, ABs and BBs. 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

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