Tetrasomy

Tetrasomics have a particular chromosome represented in four doses. Therefore, general chromosome formula for tetrasomics is 2n + 2 rather than 2n + l+l, the latter being a double trisomic. All 21 possible tetrasomics are available in wheat. Besides these tetrasomics, E.R. Sears was also able to synthesize a complete set of compensating nullisomic tetrasomics (2n — 2 + 2), where addition of a pair of homologous chromosomes would compensate for the loss of another pair of homologous chromosomes. Such non-homologous chromosomes, which are able to compensate for each other, are considered to be genetically related and are called homoeologous chromosomes.