Frequency and characteristics of the genetic disease in a pediatric hospital1 'equency and characteristics of patients discharged with recoyn zed genetic diseases by current technical literature available in 1988 (GDI from a pediatric hospital at Santiago, Chile, are described. On the last three months of year 1983, a prospective study on 2,987 hospital admissions corresponding to 1,196 patients was done. Prevalence and incidence of GD were 62.5% and 17% among hospital acmissions as a whole and 49.75% and 47.7% respectively when only individual patients were considered. No sex di f ferences were found between patients with and without GD, but the frequency of GD was significantly higher in patients staying 7 or more weeks in hospital (12% vs. 3.4%), as among those who required two or more admissions (10.9% vs. 4.3%) or were refered from outside metropolitan Santiago (37.3% vs. b.1%). hirst ten more frequent GD, in decreasing order of importance, were cardiac malformations, leukemias, cleft lip and/or palate, congenital dislocation of the hip, epilepsy and convulsions, idiopathic scoliosis, Down's syndrome, anal malformations, aganglionic megacolon and cystic fibrosis. The importance of diseases with genetic background is thus emphasized in a pediatric hospital.

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This method has been used to verify the presence of the Y chromosome as well as screening for its numerical or structural aberrations. Bucal smear was done in 50 healthy males as controls, and in 10 cases of different sexual pathologies or deletion of the Y chromosome. The smears where stained with 0,5 % atebrin solution. The percentage of positivity and its morphological differences were found in agreement with those obtained in other countries.Casperson (1968)(1) y Zech (1969) (2, 3) observaron que la porcion distal del brazo. q del cromosoma Y presentaba una gran fluorescencia, cuando se tenia con quinacrina mostaza. Pearson y Bobrow (1970) (4) investigaron la posibilidad de identificar el cromosoma Y en nucleos interfasicos, basandose en la propiedad de tincion que este presentaba. Observaron que el material cromatinico era posible detectarlo en dicha etapa celular, tratandolo con quinacrina o sus derivados. El corpiisculo "Y" aparece como un cuerpo brillante bien delimitado dentro del niicleo, sobre un fondo oscuro, y debe diferenciarse de bacterias y otros restos cromati'nicos. Desde que se describio la tecnica de fluorescencia, se ha utilizado en el diagnostico de aberraciones gonosomicas e identification del sexo (esta ultima puede ser utilizada en diagnostico prenatal) (5).Los objetivos de este trabajo son: 1. Establecer el rango de positividad del corpusculo "Y" fluorescente en una poblacion normal, y compararlo con casos que presentan patologia sexual y/o del gonosoma "Y". 2. Analizar las caracteristicas morfologicas del .Profesor titular de Pediatn'a

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Mosaic Isochromosome 18q: case-report Background: The Isochromosome 18q and chromosome 18 short arm deletion (18p-) constitute structural anomalies that are reported with certain frequency in the literature. However, the association of both abnormalities in a patient is very uncommon. Objective: Description of a clinical case of Isochromosome 18 with emphasys in the few phenotypic manifestations. Case-report: Female infant 18 months-old, with short stature, minor dysmorphic features and a slight psychomotor developmental delay, whose chromosomal study in peripheral blood showed a chromosomal mosaicism with two cell lines: chromosome 18 long arm isochromosome and deletion of chromosome 18 short arm. The chromosomal analysis of both parents did not show numerical neither morphological alterations. Discussion: This case illustrates the importance of requesting a karyotype in patients with small stature, dysmorphic features and/or malformations. The patient clinical features are compared with other similar cases described in the literature. The coexistence of both structural abnormalities (mosaicism) is extremely uncommon.