Some chromosome rearrangements are completely harmless, as long as nothing goes missing
A chunk of one chromosome can snap off and swap places with a piece of another, and the person carrying it may never notice. What matters is balance: if every gene is still present and no gene is broken across the join, the shuffle can be harmless.
A chromosome abnormality is any missing, extra or irregular stretch of chromosomal DNA. Doctors spot them by comparing someone's karyotype, the full picture of their chromosomes, with the typical set. They fall into two broad families: problems of number and problems of structure. Most come from mistakes in cell division.
Number problems are called aneuploidy. The commonest culprit is nondisjunction, in which duplicated chromosomes fail to separate, especially when eggs are forming, leaving a sex cell with one too many or one too few. Most aneuploid people have an extra copy rather than a missing one. Down syndrome, trisomy 21, involves a third chromosome 21, while Turner syndrome is a monosomy in which a person has a single X and no second sex chromosome. Whole extra sets, as in triploid or tetraploid cells, are polyploidy. On the father's side, smoking and workplace exposure to benzene, insecticides and perfluorinated compounds raise the odds of sperm with the wrong count.
Structural problems start with breakage and faulty repair. Unbalanced ones lose or gain material: a partial deletion on the short arm of chromosome 4 causes Wolf-Hirschhorn syndrome, and a duplicated gene on chromosome 17 can cause one form of Charcot-Marie-Tooth disease. Balanced ones merely rearrange. An inversion flips a segment end to end; a reciprocal translocation trades pieces between two chromosomes; and in a Robertsonian translocation, which usually involves chromosomes 13, 14, 15, 21 or 22, two chromosomes lose their short arms and fuse into one. A chromosome can even curl into a ring.
Timing matters as well. Constitutional abnormalities, typically errors in egg or sperm, sit in every cell, which is why parents are often tested when a child has one. Others arise fresh, and around 7 percent of new mutations appear as high-level mosaicism, where one body carries genetically distinct cell lines. Acquired changes during adult life stay confined to particular groups of cells and are not passed on.
Source: Chromosome abnormality
