Citation
Helfer, Robert George (1940) A Comparison of X-ray Induced and Naturally Occurring Chromosomal Variations in Drosophila Pseudoobscura. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/jreq-0n39. https://resolver.caltech.edu/CaltechTHESIS:05212024-230410309
Abstract
Summary:
1. The distribution of 347 breaks induced by X-ray treatment in the chromosomes of Drosophila pseudoobscura was studied. The frequencies of the breaks in the different chromosomes are in the proportion to the lengths of the latter.
2. The induced breaks in the third chromosome are not distributed entirely at random. The frequency of the breaks in the heterochromatin as compared with those in the euchromatin, is much greater than would be expected on the basis of the lengths of the heterochromatic portions in the salivary gland cells, but probably smaller than would be expected on the basis of its length in the mitotic chromosomes. Within the euchromatic portions the frequency of breaks increases slightly from the proximal to the distal end.
3. Aside from the regularity mentioned in the preceding paragraph, the breaks in the third chromosome show no tendency to be concentrated around any "weak points." In any case, a comparison of the induced breaks with those observed in the naturally occurring chromosomal aberrations shows very few coincidences. None of the inversions induced by X-ray treatment proved similar to any of the naturally occurring inversions.
4. The reunion of the chromosome fragments produced by X-ray treatment is not at random, inversions being more and translocations less frequent than expected.
5. Several mosaic chromosomal observations are described. An analysis of these aberrations seems to argue in favor of the "breakage first," rather than the "contact" hypothesis of the origin of chromosomal aberrations.
6. The "breakage first" hypothesis is also favored by the observed aberration in which paternal as well as maternal chromosomes seem to be involved.
7. An aberration involving a terminal attachment, and another showing what appears to be a branched chromosome, are described.
Item Type: | Thesis (Dissertation (Ph.D.)) |
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Subject Keywords: | Biology |
Degree Grantor: | California Institute of Technology |
Division: | Biology |
Major Option: | Biology |
Thesis Availability: | Public (worldwide access) |
Research Advisor(s): |
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Thesis Committee: |
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Defense Date: | 1 January 1940 |
Record Number: | CaltechTHESIS:05212024-230410309 |
Persistent URL: | https://resolver.caltech.edu/CaltechTHESIS:05212024-230410309 |
DOI: | 10.7907/jreq-0n39 |
Default Usage Policy: | No commercial reproduction, distribution, display or performance rights in this work are provided. |
ID Code: | 16403 |
Collection: | CaltechTHESIS |
Deposited By: | Benjamin Perez |
Deposited On: | 24 May 2024 21:05 |
Last Modified: | 29 May 2024 18:14 |
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