This work presents a classification algorithm based on genetic algorithms (GAs) that discovers comprehensible IF-THEN rules, in the spirit of data mining. The proposed GA has a flexible chromosome encoding where each chromosome corresponds to a classification rule. Although the number of genes (genotype) is fixed, the number of rule conditions (phenotype) is variable. The GA also has specific mutation operators for this chromosome encoding. The algorithm was evaluated on two public domain, real world data sets (on the medical domains of dermatology and breast cancer).