Wójcik, E. and Smalec, E. 2012. Assessment of chromosome instability in geese (Anser anser). Can. J. Anim. Sci. 92: 49-57. The basic test applied in the research of chromosome instability is the test of sister chromatid exchange (SCE). It makes it possible to identify single-and double-strand DNA damage caused by genotoxic factors and those that disrupt DNA damage repair mechanisms. Fragile sites in chromosomes can be found in all organisms. They are chromosome sites showing susceptibility to breakages and discontinuities in specific conditions of cell culture and also following induction with chemical substances. Chromosome instability of Anser anser geese was assessed in the research, focussing on sister chromatid exchange and the identification of fragile sites. The mean SCE/cell was 4.75±1.00. Most SCEs were identified in the proximal part of the chromosomes. Fragile sites were also identified in the chromosomes during the research. Altogether, 138 breakages were observed in the chromosomes. Apart from identifying chromosome damage, the particular instances of damage were located in the chromosomes.