868 



SCIENCE 



[N. S. Vol. XXXII. No. 833 



part of this volume, which ought to find its 

 way into the hands of all working ecolo- 

 ogists, are the chapters devoted to the re- 

 lation of natural vegetation to crop possi- 

 bilities, the agricultural features of Maryland 

 and the forests and their products. All 

 through the volume the several authors dis- 

 cuss the influence of climate and soil con- 

 ditions on the distribution of the native 

 plants. This study is made all the more val- 

 uable, because it is based on a careful geo- 

 logic survey made by the state survey, and 

 on the splendid soil maps and detailed soil 

 study of several portions of the state by the 

 agents of the U. S. Bureau of Soils. The 

 illustrations, carefully chosen out of a large 

 number taken to show the vegetation of the 

 state, depict some of the more striking plant 

 formations. 



A list of the 1,400 species of plants collected 

 during the botanic survey, gives in a detailed 

 manner the floral richness of the state. 



John W. Harshberger 



Univebsitt op Pennsylvania 



j SPECIAL ARTICLES 



ON A MODIFIED MENDELIAN RATIO AMONG 



YELLOW MICE.^ 



This paper is based on a series of experiments 



made possible tlirough a grant from the Carnegie 



Institution of Washington, for which grateful 



acknowledgment is hereby made. 



In 1905 Cuenot called the attention of those 

 interested in the experimental study of hered- 

 ity to the fact that in his experiments he 

 was unable to obtain any homozygous yellow 

 mice. Heterozygous yellows he obtained in 

 abundance, and found that in such animals 

 yellow was dominant to all other color forms, 

 including the gray color of wild house mice. 

 This fact in itself is worthy of note, for among 

 the rodents already experimented on, mice 

 are the only animals in which the yellow coat 

 dominates black or brown. 



Cuenot found that in a total of 363 young 

 obtained in yellow X yellow crosses, 263, or 

 72.45 per cent., were yellow, and 100, or 27.55 



^ Contributions from the Laboratory of Genetics 

 of the Bussey Institution, No. 6. 



per cent., were of other colors. In view of the 

 fact that the percentage of yellows that he 

 obtained was smaller than the Mendelian ex- 

 pectation by 2.55 per cent., he tested individu- 

 ally the breeding capacity of the yellow ani- 

 mals that he had obtained. In no case was 

 he able to discover an animal which in crosses 

 with gray or black animals would give only 

 yellow young. But if any of the yellows had 

 been homozygous such a result would of course 

 have been obtained, and from the application 

 of the ordinary Mendelian principles we 

 should expect one third of the yellows that he 

 tested to have been of this sort. 



It is then perfectly certain that in his ex- 

 periments homozygous yellows were not 

 formed. With this in mind, he sought an 

 explanation of the percentage of yellows that 

 he had obtained. If the homozygous yeUow 

 class had simply been wanting entirely, he 

 should have obtained 66.66 per cent, of yellow 

 mice, and the remaining 33.33 per cent, of 

 other colors. Cuenot explained the observed 

 increase above 66.66 per cent, by supposing 

 that all of the " yellow " eggs which would 

 naturally, as a result of random unions of 

 gametes, be fertilized by yellow sperm, fail to 

 be so fertilized, but that some of them sub- 

 sequently are fertilized by non-yellow sperm 

 and so produce heterozygous yellow young. 

 The proportion of yellow young produced is, 

 accordingly, greater than two thirds but less 

 than three fourths. 



Bateson and Punnett commenting on 

 Cuenot's results, point out the fact that even 

 if two gametes bearing the character " yellow " 

 are unable to unite with each other, there 

 should, nevertheless, be no deficiency of yellow 

 young, that is, they should equal 75 per cent. 

 For suppose a yellow egg is first approached 

 by a yellow sperm. If no union of the two 

 occurs, the egg may still remain capable of 

 producing a yellow zygote, provided it pres- 

 ently meets a non-yellow sperm. But this 

 should in all cases be possible, since sperma- 

 tozoa are regularly present in excess, and the 

 spermatozoa of a yellow mouse are by hypothe- 

 sis half yellow and half non-yellow in char- 

 acter. 



