The population genetics of unequal crossing over was examined for an infinite population with random mating. The following cases were considered: 1. There is an initial portion of duplicated genes which offer the opportunity for unequal crossing over, but the primary event leading to the duplication does not occur any more (model 1a). 2. This primary event occurs with a certain (small) probability (model 1b). For both possibilities the long-term consequences for the distribution of “alleles” (i.e. the single gene, the duplicated gene, the triplicated gene etc.) were considered with the following additional assumptions: 1. No selection. 2. Selection with maximum fitness for an optimum “allele length” (i.e. number of gene repeats). 3. For model 1a, selection with general advantage of longer alleles over shorter ones was also examined.