36 Mutations and Evolution 



to each other, has been prodigious and is an achievement in itself. 

 The data for sex-linked characters alone involved the breeding of 

 over half a million flies. 



Chromosome No. 1, the X, (see Morgan et al. 1915), contains 

 all sex-linked mutations, since this is the sex chromosome. Such 

 factors already number over 50, including a series of eye colours, 

 bar eye with a reduction in the number of facets, yellow and sable 

 body colours, forked bristles, club, miniature and rudimentary wings, 

 and several lethal factors which prevent development when present 

 in the homozygous condition. Chromosome No. 2 is the longest 

 pair and contains such mutation factors as purple eye, curved and 

 vestigial wings, and black body. No. 3, the next longest, as shown 

 by linkage percentages, contains such eye colours as pink and peach, 

 rough eye due to a peculiarity of the facets, ebony, sooty and sepia 

 body colours, and beaded wings. The fourth group of factors, more 

 recently discovered, and transmitted independently of the others in 

 the Mendelian sense, contains only two known factors, bent wings 

 and the eyeless condition. It evidently corresponds with the fourth, 

 very small, pair of chromosomes. 



It has long been known that crossing over, which may occur 

 between the various members of each group, is confined to the 

 female. The amount of crossing over is also independent of the 

 way in which the factors enter an individual in a cross. Thus if 

 they enter from the same parent (i.e., in the same chromosome) 

 any two factors tend to cross over with a certain frequency, while if 

 they enter from different parents they tend with equal frequency to 

 cross over so that both will be in the same chromosome in the 

 offspring. In the male of Drosophila there is no crossing over 

 between the X and Y or between any other pairs of chromosomes. 



The probable reason for this has only recently been discovered 

 (see Doncaster 1920, p. 235) by Metz. It apparently depends upon 

 the fact that while in the egg there is a stage during maturation 

 when the long, thin chromosomes are twisted round each other, 

 there is no such stage in the male during the period preceding the 

 separation of the chromosome pairs, the chromosomes remaining in 

 a fairly condensed state during the whole growth period of these 

 nuclei. Once again cytology furnishes a basis for the peculiarities 

 in the phenomena of inheritance observed. In some other 

 organisms (moths and fowls) crossing over takes place only in the 

 male and not in the female, while in Primula, grasshoppers and rats 

 it takes place in both sexes. That crossing over involves a break 

 in the chromosome at a definite point, and not an indiscriminate 



