Aneuploidy is the presence in a cell of a typical number of chromosomes. An extra or missing chromosome is a common cause of genetic disorders including human birth defects. Wheat has an allopolyploid origin, and the homoeology existing between its three component genomes allows a range of aneuploidy to be tolerated.
The late Prof. Ernie Sears began accumulating this aneuploid set in the 1940's, and it now comprises around 220 lines. The set is primarily derived from a single cultivar, 'Chinese Spring', and features the loss of one chromosome (monosomy) or a pair (nullisomy), extra chromosomes (trisomy and tetrasomy) and compensation (nullisomy-tetrasomy) involving all 21 constituent chromosomes. There are also telosomic sets in which the same half of a pair of chromosomes is missing (derived via centromere breakage of univalent chromosomes during meiosis. Because each accession has an aneuploid genotype, it is necessary to screen cytologically at either mitosis and/or meiosis each time they are grown. The transfer of aneuploidy to agronomically adapted cultivars via traditional plant breeding methods permitted its effects to be more easily determined.