116 GENETICS [BoT. Absts., Vol. IX, 



men of height 5 ft. 9 in. than there are of height 5 ft. 8 in. or 5 ft. 10 in. He shows that this 

 is not due to the inclusion of men of different ages, since it occurs in 10 of the 13 age groups 

 (5-year groups) taken separately. He also points out that it is not due to a predilection for 

 even heights. The instance shows how difficult it is to obtain an "unselected sample" by 

 merely securing large numbers without scientific control of the original observations. — L. /. 

 Reed. 



740. Bridges, Calvin B. Gametic and observed ratios in Drosophila. Amer. Nat. 55: 

 51-61. 1921. — Lack of correspondence between gametic series and observed classes in genetic 

 work with Drosophila depends largely on 3 factors: (1) The viability of the mutant type and 

 character combination; (2) the suitability of the culture medium and other environmental 

 conditions; (3) the competition due to over-crowding. The last can be met by concentrating 

 on a few cultures in which food is provided in abundance. To meet (2), it has been found that 

 a 1 per cent banana agar (equal amounts of banana and water) gives the most satisfactory 

 culture medium for breeding large numbers of flies. For lessening or eliminating the effects 

 of poor viability of certain mutant characters or character combinations, the most successful 

 method found is that of making up "alternated stocks" such that the characters are divided 

 as evenly as possible between the 2 parents of the cross. When the Fi progeny are back- 

 crossed to the multiple mutant stock, their offspring, from which the linkage ratios will be 

 calculated, show a very small percentage of inviable combinations, and these tend to balance 

 each other. Such methods are constantly making the genetic results in Drosophila more 

 accurate^— iy. H. Plough. 



741. Cleghorn, Maude L. First report on the inheritance of visible and invisible char- 

 acters in silkworms. Proc. Zool. Soc, London 1918: 133-146. 1918. — Crossing multivoltine 

 and univoltine races resulted in producing Fi generations which differed from each other in 

 that these generations were multivoltine or univoltine depending upon the character of the 

 mother, that is, each of these generations followed the female parent. The results of num- 

 erous crossings of various kinds lead the author to conclude that the maternal parents are 

 dominant in the univoltine and multivoltine character, respectively, and that these characters 

 are inherited from the paternal grandparent in which they were dominant characters. Cross- 

 ing races differing in cocoon size resulted in producing largest average size of cocoons in every 

 3rd generation. Crossings for cocoon (silk) color show that the inheritance of this character 

 is clearly Mendelian. — Vernon Kellogg. 



742. Detlefsen, J. A. Is crossing over a function of distance? Proc. Nation. Acad. 

 Sci. [U. S.] 6: 663-670. 1920. — It is a current concept of recent genetics that genes may be 

 given definite loci on chromosome maps, on the basis of the percentage of crossing over between 

 them. It is believed that these maps roughly indicate the actual distances apart of the genes 

 in the chromosome. Certain experiments involving selection of strains of Drosophila showing 

 high and low crossing over between white and miniature in the sex chromosome throw doubt 

 on the validity of this hypothesis. The stock originally showed about 33 per cent crossing 

 over, but as a result of selection in a minus direction this percentage was reduced to (no 

 crossing over) in 10 generations in 1 line, and to 6 after 28 generations in another. These 

 stocks when inbred continued to give this reduced percentage of crossing over. Selection for 

 increased crossing over was ineffective. Crosses of a strain giving 6 per cent crossing over 

 with the original stock gave Fi females which showed a percentage about midway between 

 the two. The evidence indicates that the percentage of crossing over in this case is a "varia- 

 ble which is determined by the different possible combinations of multiple modifying factors." 

 The author concludes that "crossing over is not necessarily proportional to distance," that 

 "distance between two genes may remain fairly constant, but the amount of crossing over 

 depends on numerous hereditary factors." — //. //. Plough. 



743. Federley, Harry. Die Bedeutung der polymeren Faktoren fiir die Zeichnung 

 der Lepidopteren. [The significance of polymeric factors for coloration in Lepidoptera. ] Heredi- 

 tas 1 : 221-269. 9 fig. 1920. — The black aberration, zatima, of the arctiid moth, Spilosorna 



