Variations in Chromosome Number and Structure
The number of chromosomes in each cell of an organism is generally fixed and ranges from one in bacteria to hundreds in some plants and animals. Most organisms are diploids (2n) since their somatic cells have a chromosome complement consisting of two homologous sets. A few species, however, naturally have only one set of chromosomes, and they are called haploid or monoploid (1n). Gametes generated in diploid organisms by meiosis and gametogenesis also contain the haploid number of chromosomes. Any organism with complete sets of chromosomes that exceed two is a polyploid. The condition of polyploidy has been established surprisingly often among organisms, especially in plants. Aneuploidy, on the other hand, refers to cells or individuals that have one, two, or a few chromosomes either absent or in excess of the basic number for that species; that is, a lesser or higher number exist than found in complete sets. These differences are chromosome anomalies, and they can occur in a variety of ways. In addition, one or more individual chromosomes can be structurally altered into a variety of rearrangements. Such changes are called chromosome aberrations. Deviations from haploidy in sex cells, diploidy in somatic cells, and normal chromosome structure are not rare.
KeywordsChromosome Number Down Syndrome Reciprocal Translocation Paracentric Inversion Loop Configuration
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