Chromosomal aberrations in man

Abstract

Summary The following are the most significant points to emerge in summarizing the present status of the field of human cytogenetics. 1. The human chromosome number is 46; a standard system of nomenclature has been established for identification of somatic metaphase chromosomes. Methods, however, are not yet refined enough to detect small changes within individual chromosomes. 2. The sexual dimorphism existing in somatic cells due to the presence of sex chromatin in nuclei of females permits a rapid survey method for detecting discrepancies between phenotypic and genotypic sex. Chromatin positivity or negativity, however, should be correlated with chromosome analyses since only limited deductions concerning the sex chromosome complement can be made from the presence or absence of sex chromatin. 3. The sex chromatin body in female diploid cells is a manifestation of a single X-chromosome. A cell with 2 sets of autosomes and 2 X-chromosomes is found to have one heteropyknotic X during interphase. The XO or XY cell has no heteropyknotic X. In cells with 2 sets of autosomes and more than 2 X-chromosomes, more than one X may be heteropyknotic, permitting a single X to remain isopyknotic to the autosomes. 4. The greater number of chromosomal anomalies have been found to involve the sex chromosomes. This may reflect the heterochromatic nature of the X and Y, permitting unbalanced conditions to be viable for the organism. Only one monosomic condition, the XO, has thus far been described. No trisomic conditions for the larger autosomes have been detected. 5. The Y-chromosome is a male-determining body. The only cases in which a testis is present in the absence of a Y concern hermaphrodites. In other situations, the Y in combination with one or more X's influences development in a male direction. The female-determining potentialities of the X are not so clearly delineated as the wide range of phenotypes associated with the XO condition seems to indicate. The association of mental retardation with anomalies of the X-chromosome is unusually prevalent. 6. Clinical conditions characterized by primary gonadal failure or intersexuality have been clarified by chromosomal analyses. There is need for detailed studies of cells from many tissues to delineate the role of mosaicism, especially as it may pertain to hermaphroditism. 7. Nondisjunction, influenced by maternal age, is an important etiological basis for the existence of trisomic states. 8. For the pediatrician, the studies on Mongolism are of primary importance. The genetic aspects of the condition have been clarified and a basis for the familial occurrence emphasizes the necessity of chromosomal analyses in providing information in regard to future pregnancies in young mothers with affected children. 9. The presence of an attenuated chromosome 21 or 22, the Ph1 chromosome, is associated with chronic myeloid leukemia.