Includes bibliograpical references (leaves 119-134). The most common chromosome abnormalities in live cattle are various Robertsonian translocations (centric fusions). Two hypotheses have been used to explain how monocentric Robersonian translocation chromosomes are generated: either direct formation, or evolution from dicentic chromosomes. Four main cattle procentric Satellite sequences were used as single and two-colour fluorescence in situ hybridization probes for studying the centromere organisation of cattle autosomes and the rearrangement in two cattle Robertsonian translocation chromosomes, the t(1:29) which is monocentric and found in numerous breeds, and the t(14:20) which is dicentric and found in 2 breeds.

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The present study was initiated with the view of achieving two goals: 1) to establish a suitable genetic assay system for measuring the frequency of spontaneous and induced structural and numerical aberrations of autosomes during meiosis in females and 2) to provide a better understanding of the mechanisms responsible for the production of the aberrant classes recovered. By selective exclusion of all regular meiotic products this system enabled the recovery of large numbers of aberrant products. The multiplier system served as an internal dosimeter and provided an estimate of the population size from which the aberrancies arose which in turn provided a measure of the frequencies of each event. The four different classes of exceptional meiotic products were named according to the source or the structural nature of the chromosomes: reductional nondisjunction as "matroclinous"; equational nondisjunction as "equationals"; loss of chromosome 2 as "patroclinous"; and the attachment of homologous arms as "compounds". The results suggest that two main factors affect the recovery of induced aberrations: of most importance is isosequentiality and of lesser importance is genetic background. The three classes of simultaneously recovered progeny (excluding equational nondisjunctions) arise from a common mechanism of induction; a mechanism which also accounts for free arm formation. The location of the breaks, the position of the chromatids and the method of reconstitution determine the type of aberration produced. The reconstitution of these breaks in aberrant ways are referred to as interchanges. Furthermore, it would appear that the reconstitutions are restricted in that euchromatic breaks attach to euchromatic breaks and heterochromatic to heterochromatic. Interchanges resulting from breaks on opposite sides of the centromeres of homologues result in the formation of non-sister compound chromosomes and from breaks on opposite sides of the centromeres of sister chromatids result in the formation of sister compound chromosomes. The interchange, if between heterologues, could lead to the nondisjunction of a pair of chromosomes and be recovered, as in the present study, as matroclinous progeny. The reciprocal product of the interchange between heterologues would produce an equal number of nullo eggs observed as patroclinous progeny, but if the dyad so formed is heteromorphic, i.e. chromatids of different length, it would result in the greater recovery of patroclinous progeny because of the preferential inclusion of the shorter chromatid. The evidence for interchange mediated aberrations is provided by the recovery of free arms of chromosome 2. Experimental support for these events is provided by the unequivocal identification of the centromeres involved, which, as in this study, is made possible through the use of metacentric autosomes. / Science, Faculty of / Zoology, Department of / Graduate

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No description available.

Cell cultures were examined for percentage of fragile sites seen in straight-stained, Q-stained and reverse fluorescent-stained preparations. In all cases, percentage of fragile site expression was decreased when compared to straight-stained preparations. However, fragile sites seen in Q- and RF-stain could be identified as on X chromosomes.

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The chromosome constitution of human cleavage stage embryos has been extensively investi-gated using a variety of techniques, revealing high levels of aneuploidy and mosaicism. However, the final phase of preimplantation development, the blastocyst stage has received relatively little attention mostly because it is only recently that embryo culture has become sufficiently well optimised to reliabley generate blastocysts. One of the aims of this study was to examine blastocyst cytogenetics, characterising the extent and variety of aneuploidy and, where possible, determining the origin of the abnormalities detected. Both the frequency of aneuploidy and the incidence of mosaicism were significantly lower in the 52 embryos generated by 20 patients that had successfully undergone the first cellular differentiation, producing trophectoderm (TE) and inner cell mass (ICM). Valuable tools for the detailed chromosomal analysis of blastocysts, used in both research and clinical contexts, were comparative genomic hybridization (CGH) and array CGH (aCGH). However, validation of these methods, especially aCGH, was required in order to verify accuracy. A low error rate and a low misdiagnosis risk were demonstrated. The morphology of 1397 embryos at the cleavage and blastocyst stages from 229 patients was evaluated in relation to their chromosomal complement. The results obtained during this part of the project showed that, in general, there is little correlation between cleavage stage morphology and chromosome status. A weak link between morphology and aneuploidy, however, was found for embryos at the blastocyst stage. Chromosomally normal female embryos had a tendency to grow faster than male embryos at the cleavage stage and therefore tended to achieve superior morphological scores, whereas the trend was reversed at the blastocyst stage. Abnormal embryos carrying types of aneuploidy compatible with formation of a clinically recognised pregnancy had morphologies indistinguishable from those of euploid embryos. This study also aimed to utilise aCGH for the preimplantation genetic diagnosis (PGD) of imbal-ances due to structural chromosome rearrangements (e.g. translocations) in 39 carriers, a total of 139 embryos were assessed. The data obtained revealed that carriers of Robertsonian translocations are at increased risk of aneuploidy affecting additional chromosomes not involved the translocation, a phenomenon known as an interchromosomal effect (ICE). Finally, the clinical outcomes of 300 patients undergoing preimplantation genetic screening (PGS) using aCGH, for various different indications, were evaluated at both the cleavage (795 embryos) and blastocyst stages (1097 embryos). The pregnancy rate following cleavage stage biopsy was significantly lower than following blastocyst stage biopsy. The miscarriage rate was significantly reduced following PGS for patients with recurrent miscarriages. This work provided promising data supporting the clinical use of comprehensive chromosome analysis for the screening or diagnosis of preimplantation embryos and also yielded scientifically useful information concerning the frequency and nature of aneuploidy at the final stage of development before implantation.

published_or_final_version / abstract / Anatomy / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Biochemistry / Master / Master of Philosophy