66 LINKAGE 



factors but were of a random sort, these relations 

 could not be calculated from a linear map. 



As a concrete illustration of the way in which a 

 group of factors behaves as a linear series, attention 

 may be called to the manner of distribution of the 

 factors among the germ cells of a female heterozygous 

 for a large number of factors in the same pair of 

 chromosomes. Let us write the factors derived from 

 one parent, i.e., those in one of the chromosomes, on 

 one line (see formula p. 67), in the order which they 

 have on the map (see frontispiece), and the allelo- 

 morphic factors derived from the other parent, i.e., 

 those in the homologous chromosome, in correspond- 

 ing positions on the line below. Then in such a case 

 the mature eggs contain either all of the factors 

 represented on one line and none of those on the other, 

 or they contain all of the factors present in one section 

 of the line, and all of the factors present in the re- 

 maining section of the other line. In other words, 

 the factors obviously stick together in sections ac- 

 cording to their position in the linear series. When 

 double crossing over occurs the line is broken in two 

 places, but even here whole sections remain intact. 



The above facts may be illustrated by an actual 

 case. The first formula shows the composition of 

 a hybrid female which has received from her mother 

 the mutant factors: yellow, white, abnormal, bifid, 

 vermilion, miniature, sable, rudimentary, and forked, 

 and from her father the normal allelomorphs of these 

 factors, together with the dominant mutant factor, 

 bar. 



