378 AN INTRODUCTION TO MODERN GENETICS 



twice, so that the measurements are subject to at least this error. More- 

 over, as the chromosome is stretched, apparently single bands some- 

 times become resolved into a group of distinct bands. 



The yellow-achaete deficiency shows that several genes may be 

 present in each dark band of a salivary chromosome. It is not known 

 whether genes are also present in the non-staining regions. There may 

 also be several genes in each of the chromomeres of pachytene chromo- 

 somes, though Belling^ claimed that there was only a single one, visible 

 as a minute particle embedded in the larger chromomere. However, the 

 multiple nature of the pachytene chromomere is strongly suggested by 

 the fact that chromosomes at pachytene seem to be eight or ten times 

 shorter than in salivary glands, and in view of this the relation between 

 the size of the gene and of the pachytene chromomere is very obscure. 

 The actual diameter for the latter is from 200 to 600 mjLt. 



Estimates of the widths of the genes caimot be obtained directly from 

 measurements of the widths of the chromosomes. In saUvary glands, 

 the individual chromonemata cannot be distinguished, being pre- 

 sumably below the resolving power of the microscope. By ultra-violet 

 Hght, at least 64 parallel threads can be seen in Drosophila, and up to 

 400 in the thicker chromosomes of Chironomus ;'^ there may be even 

 more. In other chromosomes, the apparent thickness is dependent partly 

 on the spiralization and perhaps also on the amoimt of nucleic acid 

 condensed on the thread. Muller^ has made an estimate of the maximum 

 thickness of the chromonema and therefore presumably of the gene by 

 assuming that when the chromosomes are most contracted at meiotic 

 metaphase they are entirely filled with the coiled-up thread. The active 

 part of the X in D. melanogaster has a volume of about 1/12 cu. /x. The 

 length of the thread, when uncoiled, is found to be about 200 jit in the 

 saHvary glands. If the thread has a square cross section with a side x, 

 we can determine x, on the above assumptions, from the relation 



200. x^ = — , whence ;c = 20 ma. 

 12 



This again is a maximum estimate, since there is no proof that the 



chromonema does fill the whole of the metaphase chromosome, part 



of which may be occupied by accessory material. Cases are known in 



which the dimensions of metaphase chromosomes are under genetic 



control;^ for instance, in Matthiola^ there is a race with abnormally long 



chromosomes. In this case there is no very great variation in volume, 



^ Belling 1926, 1928, 1931a. ^ Bauer 1935. ^ Muller 1935c. 



* Rev. Darlington 19326, 1938. ^ Lesley and Frost 1927. 



