Copy-number fluctuation by unequal crossing-over in the chicken avidin gene family

The chicken avidin gene (AVD) forms a closely clustered gene family togethe r with several avidin-related genes (AVRs). In this study, we used fluoresc ence in situ hybridization on extended DNA fibers (fiber-FISH) to show that the number of the AVD and AVR genes differs between individuals. Furthermo re, the gene copy-number showed wide somatic variation in white blood cells of the individuals. The molecular mechanism underlying the fluctuation is most probably unequal crossing-over and/or unequal sister chromatid exchang e, as judged by the Gaussian distribution of the gene counts. By definition , an increase in gene number on one locus should be accompanied by a decrea se on the other locus in unequal sequence exchange. The results suggest tha t copy-number lability may be more common among gene families than previous ly thought. The chicken avidin gene family also provides an excellent model for studying the mechanisms of recombination and gene conversion. (C) 2001 Academic Press.