The first 22 pairs of chromosomes (called autosomes) are numbered from 1 to 22, from largest to smallest. The 23rd pair of chromosomes are the sex chromosomes. Normal females have two X chromosomes, while normal males have one X chromosome and one Y chromosome. The characteristics of the chromosomes in a cell as they are seen under a light microscope are called the karyotype. During meiosis, when germ cells divide to create sperm and egg (gametes), each half should have the same number of chromosomes. But sometimes, the whole pair of chromosomes will end up in one gamete, and the other gamete will not get that chromosome at all. Most embryos cannot survive with a missing or extra autosome (numbered chromosome) and are spontaneously aborted. The most frequent aneuploidy in humans is trisomy 16 and fetuses affected with the full version of this chromosome abnormality do not survive to term, although it is possible for surviving individuals to have the mosaic form, where trisomy 16 exists in some cells but not all. The most common aneuploidy that infants can survive with is trisomy 21, which is found in Down syndrome, affecting 1 in 800 births. Trisomy 18 (Edwards syndrome) affects 1 in 6,000 births, and trisomy 13 (Patau syndrome) affects 1 in 10,000 births. 10% of infants with trisomy 18 or 13 reach 1 year of age.Changes in chromosome number may not necessarily be present in all cells in an individual.
For example, in melanoma patients, high somatic copy number alterations are associated with less effective response to immune checkpoint blockade anti–CTLA4 (cytotoxic T lymphocyte–associated protein 4) therapy.A research work published in 2008 focuses on the mechanisms involved in aneuploidy formation, specifically on the epigenetic origin of aneuploid cells. Epigenetic inheritance is defined as cellular information other than the DNA sequence itself, that is still heritable during cell division. DNA methylation and histone modifications comprise two of the main epigenetic modifications important for many physiological and pathological conditions, including cancer. Aberrant DNA methylation is the most common molecular lesion in cancer-cells, even more frequent than gene mutations. Tumor suppressor gene silencing by CpG island promoter hypermethylation is supposed to be the most frequent epigenetic modification in cancer cells. Epigenetic characteristics of cells may be modified by several factors including environmental exposure, deficiencies of certain nutrients, radiation, etc. Some of the alterations have been correlated with the formation of aneuploid cells in vivo. In this study it is suggested on a growing basis of evidence, that not only genetics but also epigenetics, contribute to aneuploid cell formation. Partial aneuploidy
The terms "partial monosomy" and "partial trisomy" are used to describe an imbalance of genetic material caused by loss or gain of part of a chromosome. In particular, these terms would be used in the situation of an unbalanced translocation, where an individual carries a derivative chromosome formed through the breakage and fusion of two different chromosomes.