LINKAGE RELATIONS IN MEN DELI SM 117 



matin threads which twist about each other and these elements are held 

 to occupy invariable loci in the chromatin thread, then the percentage of 

 crossing-over between any two loci may be taken as an indication of the 

 distance between the factors. For obviously if the chromatin thread is 

 as likely to break between any two chromomeres as between any other 

 two, then the farther apart two factors lie in the chromatin threads repre- 

 senting homologous chromosomes, the greater is the chance that crossing- 

 over will occur between them. 



The results of the application of this idea to the linkage relations 

 existing in Drosophila are shown in Fig. 54. In this chromosome map of 

 Drosophila the factors have been plotted in a linear series according to 

 their relative position in the chromosomes as determined by linkage rela- 

 tions. The evidence as yet is not sufficient to give an accurate picture of 

 the arrangement of all the factors, but the number of factors plotted and 

 the relations which they display provide further evidence of the corre- 

 spondence between the chromosomes and the factor groups. Morgan 

 has taken 1 per cent, of crossing-over as the unit for expressing linkage 

 relations. Expressed in such units the first chromosome, which contains 

 all the sex-linked factors, has a length of 66.2. The second and third 

 groups, as far as determined, have lengths of 91.9 and 85.0, respectively. 

 These lengths in general correspond fairly well to the known relative 

 sizes of the two large pairs of autosomes when compared with each 

 other and with the X-chromosomes. In the fourth group but two 

 factors are known and their loci are so close together that thus far no 

 crossing-over has been observed between them. Accordingly no definite 

 value can be fixed for their linkage relations. From a knowledge of 

 the small relative size of the third autosome Muller, at the time he an- 

 nounced the discovery of the first factor in the fourth group, predicted 

 that factors in this group would show very close linkage values. This 

 prediction has been upheld satisfactorily and it is further evidence that 

 the chromosome theory of heredity works. 



The demonstration that factors lie in a linear series in each group 

 provides a unique method of predicting the results of factor behavior. 

 Obviously if a factor is known to belong to a particular group, it is 

 possible to predict confidently that it will display independent segre- 

 gation with factors belonging to other groups. But further than this 

 when the loci of a number of factors in a given group have been plotted 

 accurately, with a new factor it is only necessary to determine the linkage 

 relations with two of the plotted loci in order to determine its locus. 

 When its locus has been determined, its linkage values with any other 

 members in the group may be predicted from its distance in units from 

 those factors. To illustrate, in Group I, if the position of miniature 

 were unknown, it might be tested with vermilion and sable. It would 



