Numerical and structural abnormalities of chromosomes both lead to phenotypic effects. The nature of the relationship between the chromosomal abnormality and the phenotype is not clear in most cases. Unlike the single gene traits, these phenotypes are the effect of more than one gene. Deletion of genetic material is more deleterious than an excess of a similar amount. There is a range of phenotypic effects within each of the disorders or syndromes due to the same chromosomal abnormality.
Further variability can result from mosaicism.
In particular, the presence of a normal cell line in the same individual can ameliorate the effects of a chromosomal abnormality, sometimes to the extent that the phenotype appears normal. The phenotypic features which are found in the various conditions due to different chromosome abnormalities are not specific for any one abnormality. However, when a certain group or constellation of features occurs commonly in persons with the same cytogenetic abnormality a clinical syndrome can be established. Many deletions, both microscopically visible and submicroscopic, have been identified which produce recognizable clinical syndromes. The phenotype in individuals with these deletions seems to be the result of loss or interruption of several contiguous genes along the involved chromosome.