CHROMOSOMAL ABEIiRA TIONS 



201 



\arious f];cnomes and Renner complexes present in the genus interact 

 wlien brought together by hybridization. 



Duplication. — Occasionally a chromosome comes to have a certain 

 portion represented two or more times instead of once. This is known as 

 duplication. The extra portion or portions may lie next to the one 



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Fig. 15U. — Diagram illustrating the duplication (of the cd region) and the synapsis of the 

 chromosome cari-ying it with a normal chromosome. 



normally present or at some distance from it. Their orientation may be 

 normal or inverted, depending upon the manner in which they were 

 added. Evidently they originate by one or more translocations (Fig. 

 150). The presence of duplications has in some cases been revealed by 

 genetical data, and in salivary-gland chromosomes they can be readily 

 seen and correlated with abnormalities in 

 characters and breeding behavior. A sig- 

 nificant point regarding duplication is that 

 it may produce a genetical effect like that 

 of gene mutation. The dominant mutation 

 known as bar eye in Drosophila has turned 

 out to be a result of duplication (Fig. 151). 

 Aberrations and the Nature of the 

 Gene. —The chromosomal aberrations de- 

 scribed briefly in these pages are leading 

 toward a needed improvement in our under- 

 standing of the gene. For many years the 

 gene concept has been of inestimable value 

 in the task of reducing to order the multi- 

 farious data of genetics. The theory that 

 the organism has within it discrete units 

 with a special role in the inheritance and 

 development of characters rests upon the 

 independent "Mendelizing" of various small character differences in 

 sexually reproducing organisms, and upon the further fact that different 

 characters can be correlated with the activity of definitely localized small 

 regions in the chromosomes. Evidence for the occasional mutation of 

 the units is found in the sudden alterations of characters ascribed to them. 



BAR 



NORMAL 



Fig. 151. — Portion of salivary- 

 gland chromosome of Drosophila, 

 showing the duplication respon- 

 sible for the bar-eye mutation. 

 {After T. S. Painter.) 



