A translocation is a change in the arrangement of genetic material, altering the location of a chromosome segment. The most common forms of translocation are reciprocal, involving the exchange of chromosome segments between two non-homologous chromosomes. Wheat varieties are often differentiated by structural changes  of the   genome such as reciprocal translocations, deletions, inversions,  duplications  or heterochromatin polymorphisms.

The translocations are mostly identified after crossing analysis and karyological studies. Critical F1 hybrids show multivalent interchange configurations  with different frequencies per pollen mother cell. Translocations breakpoints are found interstitially or near the centromere. Centromeric translocations significantly prevailed in tetraploid wheats, while in hexaploid wheat centromeric and interstitial translocations are more frequent.Meanwhile, there  are quite  a number of cultivars in wheat characterized by the  presence of reciprocal translocations and/or their absence.

More than 70 variants of reciprocal translocations, were identified. Most are rare, while only few types had broad distribution. In common wheat, the 5B-7B translocation is the most frequent chromosomal rearrangement (cf. Tab.1).

It was found in 31 cultivars from seven countries, and 26 more cultivars were described by  SCHLEGEL (1996) as well as FRIEBE and GILL (1994).

Figure 2: Meiotic spread of F1 hybrid between hexaploide wheat, var. Chinese Spring x Poros, 2n=6x=BBAADD, after FEULGEN staining, with 4 univalents + 17 bivalents + 1 quadrivalent (= heterozygous 7B/2D translocation)

Figure 1: Mitotic spread of hexaploide wheat, var. Chinese Spring, 2n=6x=BBAADD, after N-banding

High frequency of the 5B-7B translocation can be due to the adaptive value of this rearrangement as the chromosomes 5B and 7B carry some important genes controlling plant growth and development, such as, vernalization response (Vrn1, Vrn2, Vrn3), frost resistance (Fr1), hybrid necrosis (Ne1), chromosome pairing (Ph1), resistance to Septoria leaf blotch (Stb1, Stb8), yellow rust (Yr2, Yr3,Yr6, Yr19), and others.

At the same time, the 5B-7B translocation  being abundant in Western Europe is poorly represented or absent in wheat cultivars from countries with more continental climate, such as Russia and Ukraine.

For  monosomic  analysis, chromosome identification, identity  proof of  a variety and for several other reasons the knowledge  on interchanges is required.