This study investigated the effects of selective breeding on maturity body weight (at 9-weeks of age) in Japanese quail across three generations. Descriptive statistics demonstrated a consistent upward trend in maturity body weight, with the mean increasing from 311.08 g in the first generation to 344.41 g in the third. Heritability estimated for growth traits suggested moderate potential for genetic improvement and the selection response of 18.38 g from Generation 1 to Generation 2 and 14.95 g from Generation 2 to Generation 3 indicated that the average body weight of the selected individuals increased by these amounts. The results showed that, from Generation 1 to Generation 2, the selection differential of 18.38 g reflected that the selected quail were on average 18.38 g heavier than the population mean from Generation 1. As the mean of maturity body weight increased from 311.08 g in the first generation to 344.41 g in the third, the corresponding estimated breeding values (EBVs) also reflected genetic gains, with values of 18.38 g and 14.95 g for generations two and three, respectively. The findings also identified a specific tandem repeat sequence, "CTCTCTCTCT," within the insulin-like growth factor-1 (IGF-1) gene, potentially serving as a genetic marker for predicting maturity body weight. The findings underscore the complexities of breeding strategies, as increased variability may complicate population management despite gains in mean maturity body weight. Overall the findings underscore the importance of balancing selection intensity and genetic diversity to sustain long-term genetic progress while optimizing maturity body weight traits in Japanese quail.