Chromosome variation and gene action

The causal relationships responsible for the association of phenotypic variation with chromosomal aberrations are poorly understood. An approach to the study of the primary consequences of chromosomal imbalance is the search for deviations at the level of appropriate gene products. Recent progress in gene mapping combined with the present techniques of linear differentiation of the chromosomes has improved considerably the prospects to establish relevant attributions. There are a number of gene dosage effects observed in chromosome duplications and deficiencies of various autosomes as well as of the sex chromosomes which are summarized and briefly discussed. The gene products studied so far include almost exclusively enzyme proteins. Other parameters examined are the nucleolus organizer regions which are highly variable, indicating variation in the number of ribosomal genes. In this system, compensatory regulation in the synthesis of rRNA seems to interfere with the expression of gene dosage effects. Furthermore, the influence of chromosome variation on cell kinetic parameters is considered which may result in developmental disturbances. A clue to the understanding of the effects of chromosomal imbalance is expected to come from the identification and mapping of the genes coding for cell surface antigens involved in morphogenesis.