182 FUNDAMENTALS OF CYTOLOGY 



The solution of the problem of gene position, like that of numerous 

 other cytogenetical questions, is being greatly facilitated by the use of 

 abnormalities in chromosome behavior. In the next chapter, abnormali- 

 ties of various types vnW be described. Our present purpose will be 

 served by one of them, deletion. Occasionally there appears spon- 

 taneously or in experimentally treated material a chromosome with a 

 portion missing. This portion may have been deleted from the end 

 (terminal deletion) or from some other region (intercalary deletion). 

 Most deletions render a monoploid cell inviable. The same is true of a 

 diploid cell if both chromosomes of the pair carry the deletion, but if 

 only one member of the pair carries it the cell is often functional. By 

 observing the characters affected by the absence of a chromosome 

 portion, it can be inferred what genes were lost at the time of deletion. 

 Obviously the most useful deletions are those which are small and which 

 do not prevent the development of the cells concerned. Two examples 

 illustrating the procedure will now be described. 



In maize there is a mutant gene {yg^} in the linked group occupying 

 chromosome 9. Plants carrying this gene in the homozygous recessive 

 condition {ijg^ yg^) are yellow-green, while homozygous dominant 

 {Ygi Yg^ and heterozygous {Yg^ yg^) plants have the normal green color. 

 In some strains of maize, chromosome 9 has a small terminal knob, while 

 in other strains the knob is large; hence the chromosome can be easilj' 

 distinguished in certain stages of the nuclear cj^cle. The position 

 of the yellow-green gene in the map already made on the basis of linkage 

 relationships was near one end. That it was also near the end of the 

 actual chromosome was demonstrated in the following cross (Fig. 129). 



A yellow-green plant homozygous for yg^ and a small knob was used 

 as a pistillate parent, whereas the staminate parent was normal green 

 and homozygous for Yg2 and a large knob. Pollen from the latter plant 

 was given X-ray treatment, which is known to induce chromosomal 

 aberrations, and then placed upon the silks of the pistillate parent. 

 When the resulting kernels were sown, they produced a population com- 

 prising plants of several classes. Most of them were normal green in 

 color, as expected in heterozygotes, and cytological examination showed 

 the members of their ninth pair of chromosomes to have been unchanged 

 by the irradiation. Plants of a second class were yellow-green and had 

 the large-knobbed chromosome (from the pollen parent) considerably 

 shortened, evidently by the deletion of a region near the knob. A third 

 class, also yellow-green, had the large knob reduced in size, indicating 

 a deletion including a portion of the knob. In all cases the maternal 

 chromosome remained unaltered. These cytological and genetical data 

 showed that the deletions had removed the dominant normal gene, 

 Yg2, from chromosome 9 in some of the pollen and also that the gene 



