Molecular studies of chromosome 18

Fisher, Julia Marilyn (1994) Molecular studies of chromosome 18. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The parental origin of the additional chromsome 18 in 63 trisomic conceptions was maternal in 61 (96.8% ) cases and paternal in 2 cases (3.2% ). Both the paternal cases were consistent with a postzygotic mitotic (PZM) error. Amongst the informative maternal cases, 16 (29.6% ) were due to meoisis I (MI) errors, 35 (64.8% ) to meiosis II (MII) errors and 3 (5.6% ) to PZM errors. Five of the MI errors were nullichiasmate. A standard chromosome 18 linkage map was constructed and a comparison made with the centromere maps generated from the nondisjoined chromsomes in maternally derived trisomy 18. Nullichiasmate meoisis and a mechanism unassociated with recombination, both of which are maternal age dependent, were found to play the major roles in the aetiology of trisomy 18.

Trisomy 18 shows a similar proportion of maternally derived errors as other trisomies, such as those of chromosomes 13, 16, 21 and 47,XXX individuals (Hassold et al. 1987); Sherman et al. and MacDonald et al. unpublished observations), but is unique in showing a relative excess of MII errors. Chromosome 18 appears to share some similar maternal nondisjunctional mechanisms with chromosomes 21 and the X.

Observations involving chromsomes 16, 21, 18 and the X, therefore show that there are a wide variety of mechanisms involved in the nondisjunction of human chromsomes. The parental origins of several unbalanced de novo structural abnormalities of chromosome 18 were also determined. A literature search established these results to be consistent with observations for other chromsomes. Additionally, 19 chromosome 18 loci were physically mapped to particular regions of the chromosome using the unbalanced structural abnormalities.

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