Placental and fetal tissues from 46 human pregnancies were cultured and cytogenetically analyzed in an attempt to document the existence of chromosomal mosaicism confined strictly to tissues of extraembryonic origin. In two gestations in which chromosomal mosaicism was found, it was expressed exclusively in placental chorionic cells and was not detected in cells derived from the embryo proper. This demonstration of confined chorionic mosaicism may have implications for the understanding of the fetoplacental unit and for prenatal diagnosis.

Confined chorionic mosaicism, detected commonly on chorionic villus sampling (CVS) and occasionally in cultured amniotic fluid cells, is described in five pregnancies that showed confined chorionic mosaicism for trisomies 12, 13, 14, 17 and a marker chromosome. Cytogenetic findings in these pregnancies support the conclusion that within chorion some chromosomal mosaicism are confined to the trophectoderm derivatives while others to the extra-embryonic mesoderm. The etiology of confined chorionic mosaicism is discussed in relation to a significant role of multiple cell lineages contributing to the early development of placenta. The need is indicated for the use of both direct and long-term cultures in CVS prenatal diagnosis, and for the confirmatory testing of fetal blood or amniotic fluid in cases where mosaicism is detected in chorionic villi.

The syndrome of tetrasomy 18p has been well documented in the literature. This is typically a result of a supernumerary isochromosome 18p, that has arisen during maternal meiosis II. This report presents clinical and molecular findings in two maternal half sisters with an isochromosome 18p. The isochromosome is inferred to have arisen during meiosis in the maternal grandmother and to have undergone mitotic and meiotic recombination in the mother of JJ and AT. The abnormal cell line may be restricted to the gonad in the mother as only normal 46,XX cells were detected by cytogenetic analysis of her blood or fibroblasts and physical examination revealed only normal findings. Thus, the isochromosome, although present at fertilization, must have been lost from the majority of embryonic precursor cells. This case raises important genetic counseling issues concerning recurrence risks.

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Chromosomal, clinical, and psychological data are presented on members of six families with X-linked mental retardation. Affected males in three of these families express the fra(X)(q28) marker, while the retarded males in the other three do not. Similar variable physical and psychological characteristics, such a lop ears, large testes, and perseverative speech, are present in affected males in all six families. Preliminary analysis of the psychological data also shows that males with and without marker expression cannot be differentiated with certainty. On this basis we suggest that there is a type of X-linked mental retardation with many phenotypic features of marker-X mental retardation but without expression of the X chromosome fragile site.