From Polytene Chromosomes to Human Embryology: Connections via the Human Fragile-X Syndrome

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Abstract
Inferences about one aspect of human embryology came from analysis of the obscure cytogenetic phenomenon of intercalary heterochromatin in Drosophila melanogaster. The bridge between these two topics is a human syndrome that represents the most common cause of inherited mental retardation, the fragile-X syndrome. Two models were developed to make this bridge. One model uses intercalary heterochromatin to provide an explanation for the basis of human fragile sites. The other model presents an explanation for the complex pattern of inheritance and expression of the fragile-X syndrome. This syndrome is proposed to result from abnormal chromosome imprinting. The imprinting event gives a potentially mosaic population of primary oocytes in females who inherited a non-imprinted fragile-X chromosome. This mosaicism provides a method to estimate the number of oogonial progenitor cells in humans. This example illustrates the unpredictable course of basic research, and emphasizes the usefulness of maintaining for such basic research adequate research funding from biomedical research budgets.

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