Aneuploidies means that there is an abnormal number of chromosomes in a cell. Remember that normally there are two chromosomes in each cell. The types of conditions that occur depend on the number of chromosomes in a cell. Monosomy, which is one chromosome in a cell, causes Turner syndrome if the defect occurs on the sex chromosomes. Trisomy, which means that there are three chromosomes in the cell, can cause different conditions depending on which chromosome is affected. If trisomy occurs on the 21st chromosome, Down syndrome will occur. If the 18th chromosome is affected by trisomy, Edwards syndrome will occur. Finally, if trisomy occurs on the 13th chromosome, Patau syndrome will occur. Tetrasomy and Pentasomy, which is four chromosomes in one cell and five chromosomes in one cell, are very rare, but have occurred in humans. This can result in XXXX, XXYY, XXXXX, XXXXY and XYYYY syndromes.
Variations in chromosome number involving individual chromosome are included in aneuploidy. Aneuploid plants and animals have incomplete genomes. Individual chromosomes may either be less than the diploid number (monosomics and nullisomic), or more than the diploid number (polysomics).
Aneuploidy may arise by two methods. (1) There may be a union of gametes of which one contains an unbalanced number of chromosomes. (2) It may also arise by the perpetuation of a cell in which, through a failure of anaphase of mitosis, one or more chromosomes may be added or lost.
Aneuploids rarely survive, and are much less viable than euploids. The loss or gain of individual chromosomes upsets the balance, and normal development is not possible.