Aneuploidy Development of Aneuploids Phenotypic Effects of Aneuploidy Genetics of Aneuploidy Monoploidy Euploidy Euploidy and Plant Speciation Variation in Chromosome Number WWW Links Genetic Topics

Aneuploidy Changes in chromosome number can occur by the addition of all or part of a chromosome (aneuploidy), the loss of an entire set of chromosomes (monoploidy) or the gain of one or more complete sets of chromosomes (euploidy). Each of these conditions is a variation on the normal diploid number of chromosomes. As you would expect each of these can have drastic effects on phenotypic expression. Aneuploidy - the abnormal condition were one or more chromosomes of a normal set of chromosomes are missing or present in more than their usual number of copies Monoploidy - the loss of an entire set of chromosomes Euploidy - an entire set of chromosomes is duplicated once or several times The different conditions of aneuploidy are: Nullisomy - the loss of both pairs of homologous chromosomes; individuals are called nullisomics and their chromosomal composition is 2N-2 Monosomy - the loss of a single chromosome; individuals are called monosomics and their chromosomal composition is 2N-1 Trisomy - the gain of an extra copy of a chromosome; individuals are called trisomics and their chromosomal composition is 2N+1 Tetrasomic - the gain of an extra pair of homologous chromosomes; individuals are called tetrasomics and their chromosomal composition is 2N+2 In addition to these conditions, more than one pair of homologous chromosomes may be involved. For example, a double monosomic is missing one chromosome from each of two pair of homologous chromosome (designated 2N-1-1), and a double tetrasomic contains an extra pair of two pairs of homologous chromosomes (2N+2+2). In addition, to variation in whole chromosome numbers, genetic stocks have been developed, especially in plants, where parts of chromosomes are retained. One example is telocentrics which are chromosomes that have a terminal centromere. These structures represent chromosomes that are missing the genetic material beyond that centromere. (Stocks containing these types of chromosomes are called monotelosomics or monotelos for short.) Another type of structure is the isochromosome which is a chromosome that contains the same genetic material on both arms. (Genetic stocks which contain these chromosomes are called monoisosomics or monoisos for short.) Copyright © 1997. Phillip McClean