Drosophila melanogaster, the fruit fly, is an excellent organism for genetics studies because it has a short life span, produces large numbers of offspring, and has many types of hereditary variations that can be observed with low-power magnification. Mendelian traits studied were bar eye (B), scarlet eye color (st), and yellow body color (y). Two crosses, (bar × yellow) and (bar × scarlet), the corresponding F1 and F2 genera­tions were created according to the mating maps. The goodness of fit test for the observed data against the theoretical segregation ratios was analyzed using χ2 statistical test. The results showed that the observed segregation ratios for male and female in both crosses didn’t fit the theoretical ratios because the χ2 values were much greater than the critical χ2 value (7.82) at 5% significant level. The causes of the disagreement could be the gene interaction and the role of modifier genes. For Bar gene, the penetrance was complete in male for both crosses. This was due to the hemizygote status of the gene. However, the incomplete penetrance and variable expressivities were observed in female for both crosses. In bar × yellow, when Bar was in homozygotic status, the penetrance was 46.5% under wild type background of body color and 55.6% under yellow body color background. In the heterozygotic status, the variable expressivities were 47.1% dent eyes and 6.4% oval eyes with wild type allele, whereas 38.9% and 5.6% with yellow mutant. In bar × scarlet, the Bar gene in homozygotic status showed 64.2% and 10.5% penetrance under wild type allele and yellow allele. In heterozygotic status, the variable expressivities appeared 22.0% dent eye and 13.8% oval eye with wild type background, whereas 79.0% and 10.5% with scarlet allele respectively. The genetic study is still an effective way to investigate the gene interaction.