No. 1, August, 1920] GENETICS 57 



[_';;. MORGAN, T. II. Contributions to the genetics of Drosophila melanogaster. IV. 

 A demonstration of genes modifying the character "notch." Carnegie Inst. Washington Publ. 

 27s. P. 343-88S. 1 pi. ,15 Jig. Washington, D. C. 1919.- Notch is a dominant sex-linked gene 



affecting wings, lethal when homozygous; consequently all notch flies are female and heterozy- 

 gous. Mass selection in the direction of slight notching, carried out through 24 generations 

 of Drosophila melanogaster, resulted in marked change in direction of selection. Extreme 

 selected females, out-crossed to wild-type flies, gave ordinary notch in first generation, show- 

 ing notch gene unmodified. Linkage relations demonstrated results of selection due to re- 

 cessive modifying factor in second chromosome. Second experiment (19 generations) gave 

 similar results; crosses showed effect due to same modifier in both cases. — A modification in 

 opposite direction, called "short notch," appeared several times; outcrosses to wild flies 

 gave ordinary notch. Linkage relations showed this modification due to recessive modifier 

 in first chromosome. — Notch gene is always necessarily heterozygous, but all results show no 

 "contamination" by its normal allelomorph. Other mutations, modifying wings in somewhat 

 similar or different ways, were all located in other chromosomes or different loci in X chromo- 

 some, thus showing them independent of notch. — High sex-ratios (76:1 and 119:10), given by 

 two notch females, were undoubtedly due to lethal mutation in not-notch X chromosome, 

 as shown in other cases. Only those few sons having crossover X survive. — C. R. Plunkett. 



424. Morgan, T. H., and C. B. Bridges. Contributions to the genetics of Drosophila 

 melanogaster. I. The origin of gynandromorphs. Carnegie Inst. Washington Publ. 278. 122 

 p., 4 7^., 10 fig. Washington, D. C. 1919. — The genetic situation in Drosophila melanogaster 

 made possible experimental demonstration of causes of production of mosaics and gynandro- 

 morphs (sex-mosaics). Principal recent theories are: delayed fertilization of one cleavage 

 nucleus (Boveri 18S8) ; development from a supernumerary sperm (Morgan 1905) ; and chromo- 

 somal elimination, i.e., elimination of one X chromosome from one of daughter cells at an early 

 embryonic division (Morgan 1914). Critical evidence is obtained when gynandromorphs 

 are hybrids of known sex-linked characters, and also contain known autosomal characters. 

 A number of such cases, all described in detail, all show male and female parts differ by sex- 

 chromosome only. The elimination theory is only possible one in these cases, and covers 

 all but very few gynandromorphs in Drosophila. — Gynandromorphs start as females; a strik- 

 ing preponderance of female parts is found, as expected on elimination theory. Starting as 

 a male is theoretically possible, but not indicated in any known cases. Starting as XX 

 female, the male parts will be XO, therefore sterile (as shown in primary non-disjunction); 

 except in case of XXY (non-disjunctional) individuals, where male parts will be XY, fertile. 

 All evidence from gynandromorphs with male abdomen and testes supports these predictions. 

 — Earlier theories of gynandromorphs are critically considered. The only one besides elimi- 

 nation found necessary to employ, in a few cases, is the theory of bi-nucleated eggs. Don- 

 caster has found such eggs in Abraxas. — Both gonads of same individual are always alike; 

 which is expected if germ plasm of Drosophila arises from single cell, as in Miastor, Chiro- 

 nomus, Calliphora, and other flies. — Only one certain case was found of a somatic mosaic, 

 i.e., one not involving sex-chromosome; may be accounted for by autosomal elimination or 

 bi-nucleated egg. Rarity may be due to failure of autosomal elimination or to inviability of 

 such flies. — Ten somatic mutations described are all males, of which nine look like known sex- 

 linked characters. This is in accord with expectation, if mutation occurs in only one chromo- 

 some of a pair, as is highly probable; since visible sex-linked mutations are four times as fre- 

 quent as all dominants. Mosaics in plants are discussed; somatic mutation or chromosome 

 elimination the most probable explanations in most cases. — All known gynandromorphs of 

 Drosophila are thoroughly treated as to parentage, description, and explanation, with figures 

 and diagrams of chromosomes. The great majority are adequately explained by simple X 

 elimination, including a number from XXY mothers. Many are approximately bilateral, 

 others largely antero-posterior, some mainly female, a few mainly male, and a few very 

 irregular. In all, the male and female parts and their characters are strictly self-determining. 

 No region, however small, is interfered with by neighboring parts or action of the gonad. 

 The few cases not explicable by simple elimination are most simply explained as binucleated 



