Varieties Chinese Spring, Peko, Koga and Svenno of Triticum aestivum (L.) Thell. (2n = 6x = 42), AABBDD, were used as recurrent parents in the third backcross of the pentaploids and 34-chromosome plants obtained from the second backcross (Siddiqui 1972). The B2 pentaploids involving Chinese Spring gave a seed set of 5%, whereas the F1 and B1 pentaploids involving the same hexaploid had given 15% and 10% seed set respectively. The B2 pentaploids involving Peko and Koga both gave a higher seed set (18 and 10%, respectively) than the B2 pentaploids of Chinese Spring. Although, with successive backcrossings, the genes of the original tetraploid parent become more and more diluted, morphological resemblance of pentaploids to hexaploids might be misleading. Persistence of aestivum X durum heterozygosity would defeat the very purpose of extraction. Use of primitive tetraploids such as Triticum dicoccoides Körn, Triticum persicum (Boiss) Aitch. and Hemsl. and Triticum polonicum L., instead of Triticum durum Desf., in the production of the original pentaploids (Siddiqui 1969 a), might be a means of circumventing persistent hexaploid X tetraploid heterozygosity.
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