402 



SCIENCE. 



[N. S. VOL.XVIII. No. 456. 



dominance. It gives to the breeder a new 

 conception of 'pnrity.' No animal or 

 plant is 'pure' simply because it is de- 

 scended from a long line of ancestors pos- 

 sessing a desired combination of charac- 

 ters; but any animal or plant is pure if it 

 produces gametes of only one sort, even 

 though its grandparents may among them- 

 selves have possessed opposite characters. 

 The existence of purity can be established 

 with certainty only by suitable breeding 

 tests (especially by crossing with reces.- 

 sives), but it may be safely assumed for 

 any animal or plant descended from pa- 

 rents which were like each other and had 

 been shown by breeding tests to be pure. 



Special Cases under the Law of Mendel. 

 —It remains to speak of some special cases 

 under the law of Mendel, which apparently 

 are exceptions to one or another of the 

 principles already stated, and which prob- 

 ably result from exceptional conditions 

 known to us imperfectly. These special 

 cases have come to light in part through 

 Mendel's own work, in part through that 

 of others. 



1. Mosaic InheHtance.— It occasionally 

 happens that in crosses which bring to- 

 gether a pair of characters commonly re- 

 lated 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 

 unknoAvn, 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 off- 

 spring. The balanced relationship of char- 

 acters possessed by the parents is trans- 

 mitted to the germ-cells, which are, not as 



in ordinary hybrid individuals D or R, 

 but DR. This has been shown to be the 

 ease in spotted mice by Mr. Allen and my- 

 self, in a paper published elsewhere. 

 (Castle and Allen, :03.) 



2. Stable Hybrid i^orms.— 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— Hieracmm hy- 

 brids (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 grand- 

 parents, but breecls true to its own hybrid 

 character. This can be explained only on 

 one of two assumptions. Either the germ- 

 cells bear the two characters in the bal- 

 anced relationship, AB, as do those of 

 spotted mice, or, of the two gametes which 

 unite in fertilization, one invariably bears 

 the character A, the other the character 

 B. Of the two explanations, the former 

 seems at present much the more probable. 



3. Coupled Characters.— This is the phe- 

 nomenon of correlation of characters in 

 heredity. It is sometimes fovind that, in 

 cross-breeding, two characters can not be 

 separated. When one is inherited, the 

 other is inherited also. Thus, in cross- 

 ing different sorts of Datura (the James- 

 town weed) it has been found that purple 

 color of stem invariably goes with blue 

 color of flowers, whereas green stems are 

 constantly associated with white flowers. 

 Again in mice, rabbits and most other 

 mammals, white hair and pink eyes com- 

 monly occur together and may not be 

 separated in heredity. Very rarely, how- 

 ever, as I have observed, an otherwise per- 

 fectly white guinea-pig has dark eyes; 

 further the ordinary albino guinea-pig 



