184 



FUNDAMENTALS OF CYTOLOGY 



the basis of characteristic morphology. In the above example tlu^ 

 condition of the chromosomes was determined chiefly in the micro- 

 sporocytes, for although the chromosomes in the somatic cells of maize 

 are not large, they are beautifully displayed in a greatly extended form 

 at the pachytene stage in microsporocytes (Fig. 79). Not only do they 

 show well their minute structural features there (page 92), but they are 

 present as synapsed pairs, which makes it possible to compare minutely 

 the two parental members part for part. 



£1^ 



eyes white 



r\. 



eyes white 



W 

 eyes red 



Fig. 130. — Diagram, illustrating the method of locating genes by means of deletions in 

 saUvary-gland chromosomes. Explanation in text. (Based on drawings by 0. Mackenseji.) 



A second example illustrating this method of locating genes is taken 

 from the literature on Drosophila. The somatic chromosomes in the 

 fly are very small. Gross alterations in them can be detected, and the 

 approximate positions of certain genes have been determined by such 

 alterations. The rediscovery of the giant chromosomes in the salivary 

 glands of the larva (page 94) has now made it possible to carry on such 

 researches with far greater speed and precision. The method used in the 

 case about to be described parallels that used in the case of maize. Male 

 flies carrying dominant genes including W (normal red eyes) in their 

 chromosome I were X-rayed with the purpose of inducing deletions or 

 other aberrations in this chromosome. These flies were then mated to 

 females carrying recessive genes, including iv (white eyes, recessive), 

 in their chromosomes I (the X-chromosomes). Fi females were then 



