MENDELIAN CHARACTERS 213 



a complete account of the phenomenon of reversion 

 on crossing, an account which has already been demon- 

 strated to hold good in other instances besides that 

 of the sweet-pea. The facts are expressed in the 

 following manner by their discoverers : ' " Reversion " 

 is thus seen to be a simple and orderly phenomenon 

 due to the meeting of factors belonging to distinct 

 though complementary allelomorphic pairs, which at 

 some moment in the phylogeny of the varieties have 

 each lost their complement.'* 



Dr. G. H. Shull has classified the different known 

 types of latency in the following manner : 



1. Latency due to separation. In this form of 

 latency an allelomorph, when acting alone, has no 

 external manifestation, and is only rendered visible 

 by combination with another allelomorph belonging 

 to a distinct pair. In this way such ratios as 9 : 3 : 4 

 or 9 : 7 may arise. We have already observed an 

 example of this phenomenon in the case of the two 

 factors required to produce colour in the sweet-pea. 



2. Rare cases have been described of latency due 

 to combination. This, instead of being due to separa- 

 tion, is the result of a union in the same zygote of two 

 dominant allelomorphs, either of which alone will 

 produce a manifest character. When both allelo- 

 morphs are present the character fails to appear. 



3. Latency due to hypostasis, or the masking of 

 one dominant character by another character also 

 dominant and epistatic to the masked character, has 

 already been described in the case of the coat colours 



* Proceedings of the Royal Society, B. vol. 77, p. 238. 



