No. 3, July, 1921] GENETICS 269 



dominant growth factors. Evolutionary progress depends on advantageous variations, but 

 these are rare and their effect negligible in explaining heterosis. Practically all known maize 

 variations are detrimental, and the vast majority of them recessive. (Deleterious dominant 

 mutations are quickly eliminated.) Furthermore, present knowledge indicates that the role 

 of linkage must be of minor importance in maize; and the question is raised whether the 

 assumption of linkage is necessary to explain heterosis.— Difficulty of securing uniform strains 

 with the vigor of Fi has been assumed rather than demonstrated, since selection following 

 hybridization has never been continued long enough to approximate such homozygosity. 

 Author considers certain data of Emerson and East (1913) together with some of his own on 

 height. "In every case where a progeny was grown from a plant near the upper limit of the 

 range of the F2 its mean exceeded that of the Fi." Some of the data on length and diameter 

 of ear and length of internode also suggest isolation of strains with vigor of Fi. Tables are 

 presented showing extreme practical difficulty (due to large numbers which must be grown) 

 of obtaining homozygous Fo individuals with vigor of Fi (even without assuming linkage). 

 To obtain 1 individual 70 per cent homozygous, assuming 15 effective character pairs, would 

 require growing GOOO F2 individuals. On question of skew distribution of F2 vigor due to 

 dominance (without linkage), author presents significant table with accompanying graph. 

 With 20 character pairs involved, populations of 500 individuals would conform with normal 

 frequency curve; skewness would be detectable only in enormous F2 populations. Assump- 

 tion that relative effect of a given growth factor depends on size of organism also assists in 

 veiling skewness. Author concludes that heterosis is due to suppression of deleterious 

 recessive characters, and can be explained without assuming linkage.— MerZe C. Coulter. 



1904. Cutler, D. Ward. The cytological problems arising from the study of artificial 

 parthenogenesis. Sci. Prog. [London] 15: 435-444. 1 fig. 1921.— Review of some of the 

 evidence for (1) individuality of chromosomes, and (2) relation of chromosomes to sex, 

 preparatory to discussion of artificial parthenogenesis in later article.— .4. Franklin Shull. 



1905. Detjen, L. R. Peloria in Viola primulaefolia Linn. Torreya 20: 107-116. 10 fig. 

 1920.— A strain of violets bearing peloric and partially peloric flowers was propagated from 

 a single plant found in nature. The completely peloric flowers are symmetrical, having 4 

 sepals, 4 petals, 4 stamens, and a pistil of 4 carpels. — E. G. Anderson. 



1906. Detlefsen, J. A., and W. J. Carmichael. Inheritance of syndactylism, black, and 

 dilution in swine. Jour. Agric. Res. 20: 595-604. 1 pi. 1921.— A registered mule-foot boar 

 (syndactyl, black) crossed with registered Duroc-Jersey sows (cloven-foot, red) produced 

 only black, mule-foot pigs. The results of a back-cross with a Duroc-Jersey boar indicated 

 independent segregation of a unit factor for mule-foot dominant over cloven-foot and a unit 

 factor for black dominant over red. The red cloven-foot segregates bred true in F3 and F4. 

 The "red" segregates varied between intense red and cream. The so-called creams were 

 absolutely white at birth, merely developing traces of yellow medullary pigment later. The 

 data were insufficient to determine the mode of inheritance of the grades of intensity but con- 

 firmed earlier work indicating that white in swine often represents extreme dilution of red 

 due to factors which dilute red but which do not dilute black to a perceptible extent.— Sewall 

 Wright. 



1907. Detlefsen, J. A., and E. Roberts. Studies on crossing over. I. The effect 

 of selection on cross-over values. Jour. Exp. Zool. 32: 333-354. 2 fig. Feb. 20, 1921.— The 

 authors selected for high and low crossover values in Drosophila melanogaster . They used the 

 characters white and miniature, the genes of which lie in the X chromosome. In order to 

 save work and time single pair matings were made only in alternate generations. The other 

 generations were mass cultures. In the A series the first 5 generations of selection made no 

 change. There was considerable fluctuation. After the 5th generation progress was rapid. 

 The normal percentage of crossing over between white and miniature is 33. In F9 the authors 

 had reduced it to 16.49, From Fio to F,3 the percentage was reduced to less than 1. The 

 last generations were bred under abnormal conditions and the numbers were few. Series Ai 



