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), and white eye (w1). A cross (bar × white) and its reciprocal cross (white × bar), 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 genotypic ratios was analyzed using χ2 statistical test. The results showed that the observed ratios for male and female in both crosses didn’t fit the theoretical genotypic ratios since 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 because of the hemizygote status of the gene. However, the incomplete penetrance and varied expressivities were observed in female for both crosses. In bar × white, when Bar was in homozygotic status, the penetrance was 54.7% under red eye (wild type) allele and 0.0% under white eye allele. In the heterozygotic status, the different expressivities were 0.4% of round eye, 27.7% of dent eye, and 17.2% of oval eye with red eye allele. There was no expressivity observed with white allele. In white × bar, the Bar gene in homozygotic status demonstrated 2.6% and 1.0% penetrance under the background of red eye allele and white allele in female. The various expressivities displayed 48.5% of round eye, 39.3% of dent eye, and 9.6% of oval eye with red eye allele, whereas 78.4% of round eye, 16.6% of dent eye, and 4.0% of oval eye with white allele, respectively. In male, the Bar gene demonstrated a 100% penetrance despite the background in both crosses. A genetic study is still an effective way to investigate the gene interaction.