A condition known as mosaicism results from an error in the distribution of chromosomes between daughter cells during an early embryonic cell division, producing two and sometimes three populations of cells with different chromosome numbers in the same individual. Mosaicism involving the sex chromosomes is not uncommon. Other abnormal structural changes in the chromosome are consequences of some kind of chromosomal breakage, with either the loss or rearrangement of genetic material. Translocation involves the transfer of a segment of one chromosome to another. Inversion refers to a change in the sequence of genes along the chromosome, which occurs when there are two breaks in a chromosome and the segment between the breaks is reversed and reattached to the wrong ends.
Deletion occurs when a portion of a chromosome is lost.
When deletion occurs at both ends of the chromosome, the two damaged ends can unite to form a circle and the rearrangement produces a ring chromosome. Isochromosomes form when the centromere divides along the transverse plane rather than the normal long axis of the chromosome so that both arms are identical. All of the previously described structural abnormalities can affect both autosomal and sex chromosomes. The causes of chromosomal errors are not completely understood. In some conditions such as Down syndrome, late maternal age seems to be a factor.