236 Sturtevant. 



heterozygous female to any male, since the male-producing sperm bear no 

 sex-linked genes, and are therefore, in a sense, doubly recessive. From 

 such a cross the male offspring may always be used for direct calcu- 

 lation of the amount of crossing over. Of course, if doubly recessive 

 males be used as parents the females also ai-e available, since this is a 

 "back-cross" in the same sense as the first case given. 



As stated in my former paper, the most convenient measure of the 

 amount of crossing over has seemed to be that obtained by expressing 

 the cross-overs as percentages of the whole number of gametes. Thus 

 a "gametic ratio" (Bateson) of 3:1:1:3 would be expressed as 25° U 

 crossing over. It should be noted that the percent of cross-overs does 

 not represent the deviation from expectation, since two pairs of genes 

 lying in different chromosome pairs give 50° I ^ of new combinations. The 

 latter phenomenon, however, cannot properly be termed crossing over, 

 since it is due, not to interchange of materials between homologous 

 chromosomes, but to chance distribution of chromosomes belonging to 

 different pairs. 



Percent of crossing over is used as an index of the distance be- 

 tween any two pairs of genes. That is, the unit of distance is taken 

 as a portion of the chromosome of such length that, on an average, 

 crossing over occurs in it in one percent of the germ cells. Thus, in 

 the case of V and M, given below, there occurred 50 cross -overs in a 

 total of 1640 gametes. 3'0, the percent of cross -overs, is therefore 

 taken as the "distance" between these two loci. 



Chromosome I: Single Crossing Over. 

 Since my former paper was written Moe(jan ('13 a) has proposed 

 a new system of nomenclature, wliich I shall adopt here. Each mutant 

 is given a descriptive name, and an abbreviation of tliis name is used 

 as a symbol to represent the gene involved. If the mutant be recessive 

 the form of the gene present in it is represented by a small letter, its 

 dominant normal allelomorph by a large one. If the mutant be a do- 

 minant the letters are primed, and capitals still are put for dominants, 

 small letters for recessives. In the case of multiple allelomorphs Morgan 

 and Bridges ('13) have adopted the system of naming the two first 

 allelomorphs discovered in the above manner, and any later allelomorphs 

 are represented by adding the initial of the mutant involved as an ex- 

 ponent to the name of the locus involved. Thus white = w and its 

 normal allelomorph = W. The tliird allelomorph, eosin, is written w''. 



