Studies in cytogenetics: the somatic chromosomes, meiosis, chiasma frequency and the effect of sex on the chiasma frequencies in the Rattus norvegicus albinus
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Abstract
1. A study has been made of the somatic chromosome complex, the chiasma frequency and the relationship of sex to crossing-over and chiasma frequenc in the male and female rat-Rattus norvegicus albinus.
2. The stages of mitosis in relation to the somatic chromosomes have been discussed.
3. The number of chromosomes in the cells of both males and females have been determined as 42. which is in accord with the findings of Painter (1926, 1927), Pincus (1927) and Swezy (1928).
4. Stages of meiosis have been followed, the nature of pairing, formation of chiasmata etc. being studied throughout.
5. The chiasma frequencies have been calculated for both the male and the female in diplotene and metaphase, by interpreting the configurations in the light of the partial chiasmatype hypothesis. (Janssens 1;09, 1924 and ;Darlington 1931, 1932).
6. In addition, in the male where mitotic stages were more abundant, the chiasma frequency has bee calculated in the diakinesis stage as well.
7. It has been noticed that as meiosis progresses, the frequency of chiasma formation decreases and the terminal associations of the chromosomes increases. The apparent movement of chiasmata and the accompanying 'terminalisation' have been discussed.
S. The theories of Darlington and Sax have been considered from the presentation of configurations in diplotene and metaphase stages.
9. The various types of bivalent found have been interpreted, and the origin of various types in metaphase, from configurations in diplotene, is suggested.
10. The precocious separation of some bivalents accompanied by the tendency of others to lag has been discussed.
11. Suggestions have been made of the relation between genetical and cytological crossing -over and a parallel his been drawn between them.
12. The genetical results of various workers on the effect of sex on crossing -over has been summarised.
13. It has been found that in many forms there is a, suppression of crossing -over in one sex-usually the sex-heterozygote.
14. The results recorded in this study of the frequencies of chiasma formation, have been compared with the genetical crossing-over results
15. In common with the genetical results in rodents and in other examples, the frequency of chiasma formation is lower in the heterozygous sex - which in the rat is the male.
16. The cause of the suppression of crossing -over in the sex -heterozygote has been discussed and the results of some other workers have been summarised.
17. The forms of sex- chromosomes commonly met with have been listed.
18. The evolution of sex- chromosomes, the characters carried by them, and the genetical aspect of sex in individuals have been discussed.
19. The results given by Pincus (1927) have been compared with those found in this study.
20. The types of bivalent found have been compared and interoretatio is offered of some of Pincus's forms.
21. The chiasma frequencies have been calculated from each form and the results shown in graphical form.
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