Translocations involve exchange of material between two or more chromosomes. Problems arise with translocations when gametes from a balanced parent are formed which do not contain both translocation products. When such a gamete combines with a normal gamete from the other parent the result is an unbalanced embryo which is partially monosomic for one chromosome and partially trisomic for the other. Sometimes individuals are found who have both normal and abnormal cell lines. These people are called mosaics and in the vast majority of these cases the abnormal cell line has a numerical chromosome abnormality.
Structural mosaics are extremely rare.
The degree to which an individual is clinically affected usually depends on the percentage of abnormal cells. A routine Cytogenetic analysis typically includes the examination of at least 15-20 cells in order to rule out any clinically significant mosaicism. These are just some of the more common abnormalities encountered by a Cytogenetic Laboratory. Because the number of abnormal possibilities is almost infinite, a Cytogeneticist must be trained to detect and interpret virtually any chromosome abnormality that can occur. Down Syndrome, a common numerical abnormality.