Thesis (doctoral)--Lund University, 1994. / Added t.p. with thesis statement inserted.

Thesis (doctoral)--Lund University, 1994. / Added t.p. with thesis statement inserted.

No description available.

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The effect of different treatments on crossing over between and within genes at the tip of the X chromosome of Drosophila melanogaster was studied to determine whether exchange in the two regions occurs by different mechanisms. In response to autosomal inversions, ɣ -radiation, and heat shock, crossing over of both types was altered in the same direction and to a comparable extent. This would be expected if there were only one crossover mechanism involved. There was some difference in response of interlocus and intralocus crossing over after mitomycin C injection but, since the effect on interlocus crossing over in two separate regions was not consistent, the significance of this result is questionable. Although double crossing over involving the two interlocus regions was very rare doubles involving an inter and an intralocus region were recovered. The association of exchanges within a gene with a crossover between genes might indicate that there are two noninterfering mechanisms, or that multiple exchanges occur in short effectively paired regions. A further experiment designed to detect switch regions is outlined. / Science, Faculty of / Zoology, Department of / Graduate

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Studies on chromosomal segregation in higher organisms have established a definite relationship between crossing-over and nondisjunction -- the two appear to be inversely correlated. Most of the experiments performed involve segregation from inversion and translocation heterozygotes. [...]

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It has been suggested (Suzuki, 1965a) that the extrinsic and intrinsic factors which increase crossing over in the centromere regions of Drosophila do so by inactivating these regions at the time of crossing over; the inactivation resulting in altered chromosome structure which permits the intimate synapsis necessary for crossing over. This hypothesis predicts that the 3L heterochromatic marker w⁺ carried in Dp(wm)264.58a, which exhibits position effect variegation, would tend to be inactivated by chromosome aberrations, known to increase crossing over. The reversed acrocentric (RA) compound X chromosome, the X chromosome inversions sc⁴sc⁸ and sc⁸, the autosomal inversions Cy, Sb, and Ubx, and the Minute mutants M(2) and M(3) were tested for their effects on the expression of w⁺ in various coisogenic stocks. The amount of pigment in each eye was visually scored into twelve classes. Crossover analyses of the centromere regions of chromosomes 2 and 3 were performed to confirm the effects of sc⁴sc⁸, Cy, Ubx, and Sb on crossing over. The RA, sc⁴sc⁸, Cy, and Ubx all increase crossing over and significantly depress the activity of w⁺. The degree of pigment reduction is correlated with the amount of increase in crossing over. Combination of these aberrations produces an effect on pigmentation and crossing over greater than that produced by either considered singly. Sb has a slight effect in increasing crossing over and decreasing pigmentation. M(3), known to increase crossing over, significantly decreases pigmentation whereas M(2) does not. The effect of the X chromosome inversion sc⁸ is doubtful. Expression of w⁺ is affected similarly in males and females, an observation suggesting that the chromosome physiology of both sexes is similar although the actual factor(s) mediating crossing over are absent in the males. These results lend support to the proposed hypothesis but, with fluctuating parental source effects and variations found within a stock testifying to the general lability of the system, further tests under stringently controlled conditions are necessary before such experiments can be considered critical. / Science, Faculty of / Zoology, Department of / Graduate