The meiotic and mitotic behavior of regenerated plants derived from a long-term callus culture, designated 12-F, was analyzed. This culture was heterozygous for an amplification of the heterochromatic knob on the long arm of chromosome 7 (K7L). We aimed to investigate if the amplification resulted from a breakage-fusion-bridge (BFB) cycle or from unequal sister chromatid recombination. Therefore, C-banded mitotic metaphases and pachytene, diakinesis, and anaphase I of regenerated plants were analyzed. Additionally, the occurrence of alterations in K7L was investigated in C-banded metaphases from short-term callus cultures derived from lines related to the donor genotype of the 12-F culture. As a result, plants homozygous and heterozygous for the amplification were detected. Meiosis was normal with few abnormalities, such as a low frequency of univalents at diakinesis. In the callus cultures a chromosome 7 with knobs of different sizes in the sister chromatids was detected and interpreted as a result of unequal crossing over. Other chromosomal alterations were consistent with the occurrence of BFB cycles. The finding of unequal crossing over in the cultures supports the conclusion that the amplification in the culture 12-F would be derived from this mechanism. If the amplification was derived from a BFB cycle, the terminal euchromatic segment between knob and the telomere would be deleted, and possibly, homozygous plants would not be viable.