178 FUNDAMENTALS OF CYTOLOGY 



somal thread and that the significance of longitudinal chromosome 

 division lies in the division of these numerous units. The confirmation 

 of this view during the twentieth century has involved the answering of 

 two questions: "In what particular order are the various genes arranged?" 

 and "What is the precise location of each gene in the chromosome?" 



The determination of the serial order of the genes has been made 

 possible mainl}^ through studies on recombination. Soon after Morgan 

 and his coworkers had their intensive investigations of inheritance in 

 Drosophila well under wa}^, certain cases of recombination of linked 

 characters were observed in the flies. It was then (1911) suggested 

 that this might be a result of chromatid exchange (crossing over) such 

 as had been reported for salamander chromosomes by Janssens (1909). 

 This hypothesis was employed with notable success in the further develop- 

 ment of cytogenetics, although the complete proof of its correctness was 

 not available until 1931. The determination of serial order was based 

 upon the fact that different couples of allelic character pairs yield recom- 

 l)inations in different percentages. The hypothesis was advanced that 

 the frequency of crossing over simply varies with the distance separating 

 the two gene pairs concerned, exchanges occurring with uniform frequency 

 throughout the length of the tetrad. For convenience it was assumed 

 that when recombinations appear in 1 per cent of the individuals in F^ 

 after a testcross the two gene pairs are separated by one "crossover 

 unit" in the threads. On this basis a diagram, or "linkage map," was 

 gradually built up showing the arrangement of the various genes as 

 indicated by recombination percentages. The same procedure has since 

 been followed in the case of a number of other animals and plants. The 

 method used in constructing such maps is illustrated in the following 

 example from maize. 



Among the mutant characters known to be linked and assignable to 

 chromosome 4 in maize are the following: sugary endosperm (sui), which 

 is recessive to the normal starchy endosperm (Siii) ; tunicate ear (each 

 kernel with a pod) (Tu), dominant to normal ear (tu); glossy leaf (gh), 

 recessive to normal leaf surface (Gh). When plants heterozj^gous for 

 all three pairs are testcrossed to plants homozygous and recessive for all 

 three, the results are as follows (Fig. 126). The recombination per- 

 centage of sugary and tunicate is 29; hence these two genes are placed 

 29 units apart on a line representing the chromosome. Between 

 tunicate and glossy the recombination percentage is 11; hence gh is 

 1 1 units from Tu, but on which side? This is decided by the recombina- 

 tion percentage given by sugarj^ and glossy, which is 34: gh must 

 therefore be located to the right of Tu. In this way the relative order 

 of these three genes in the chromosome is determined. Recombinations 

 between further characters likewise indicate the positions of other genes. 



