542 PROCEEDINGS OF THE AMERICAN ACADEMY. 



1. Mosaic inheritance. It occasionally happens that in crosses which 

 briug together a pair of characters commonly related as dominant and 

 recessive, the two characters appear in the offspring in patches side by 

 side, as in piebald animals and parti-colored flowers and fruits. The 

 normal dominance apparently gives place in such cases to a balanced 

 relationship between the alternative characters. What conditions give 

 rise to such relationships is unknown, but when they are once secured 

 they often prove to possess great stability, breeding true inter se. This, 

 for example, is the case in spotted mice, which usually produce a large 

 majority of spotted offspring. The balanced relationship of characters 

 possessed by the parents is transmitted to the germ-cells, which are, not 

 as in ordinary hybrid individuals D or R, but £ DR. This has been 

 shown to be the case in spotted mice by G. M. Allen and myself, in a 

 paper published elsewhere. The balanced condition of D and R, which 

 exists in the gamete, is upset when that gamete unites with a pure R 

 (and probably also when it unites with a pure D) ; for spotted mice bred 

 to white mice regularly gave only uniformly gray or black individuals, 

 after the formula h DR -\- R = D(R).* But an exceptional spotted 

 male, own brother to those which gave the described result, apparently 

 produced gametes D and R as well as others £ DR, for by white females 

 he had pure white offspring as well as those which were gray or black in 

 color. This result can be expressed by the formulae : — 



Sperm. Ovum. Offspring. 



\ DR -j- R = D(R)* | ordinary gray or 

 D -j- R = D(R) j black hybrids. 

 R -\- R = R, white (pure). 



2. Stable hybrid forms. This is a case, in some respects similar to the 

 last, which was familiar to Mendel (:70) himself. It sometimes happens, 

 as we have seen, that the hybrid has a form of its own different from that 

 of either parent. To such cases the law of dominance evidently does not 

 apply. In a few cases — Hieracium hybrids (Mendel), Salix hybrids 

 ( Wichura) — it has been found that the hybrid form does not break up 

 in the second generation and produce individuals like the grandparents, 

 but breeds true to its own hybrid character. This can be explained only 



* Observations made since the foregoing was written indicate that the offspring 

 in this case are, sometimes at least, ^D(R) ■ (R). For when an individual of this 

 sort forms gametes, they apparently are, not pure D and R, but £ DR and R respec- 

 tively. This hypothesis accounts for the reappearance of spotted mice after their 

 disappearance for a generation in consequence of crossing. 



