Sex chromosome abnormalities occur once in every 300-400 births. In mothers over 35 this frequency is even greater, about 1 in 250. Not all of these chromosomal changes are associated with disease, but generally autosomal abnormalities tend to be more severe than sex chromosomal abnormalities and deletions more severe than duplications. Or from failure of one of the maturation divisions of either the egg or the sperm, producing a diploid gamete. A proportion of polyploid cells occurs normally in human bone marrow, as megakaryocytes usually have 8-16 times the haploid number.
Tetraploid cells are also a normal feature of regenerating liver and other tissues.
The cause of meiotic non-disjunction is not known, but occurs at increased frequency with increasing maternal age, maternal hypothyroidism and possibly after irradiation or viral infection or as a familial tendency. The cause of mitotic non-disjunction is also unknown but no predisposing factors have yet been identified. Aneuploidy can arise at either meiosis or mitosis, and meiotic non-disjunction can occur at either the first or the second meiotic divisions. At the second division then both the normal and the extra copy of that chromosome will be either maternal or paternal in origin. These all result from chromosome breakage.