Students tend to learn genetics by rote and may not realise the interrelationships in daily life. Because reasoning abilities are necessary to construct relationships between concepts and rote learning impedes the students' sound understanding, it was predicted that having high level of formal reasoning and adopting meaningful learning orientation would improve students' understanding of genetics concepts. To test this hypothesis, relationships among students' understanding of genetics concepts, reasoning ability and learning approaches were investigated. A total of 586 eleventh-grade students participated in the study. Students' reasoning ability and learning orientation were measured using the Test of Logical Thinking and the Learning Approach Questionnaire, respectively. Students' understanding of fundamental genetics concepts was assessed using the Two-tier Genetics Concepts Test. Regression analyses were conducted to examine predictive influence of reasoning ability and learning orientations on students' understanding of genetics concepts. The results revealed that all cognitive variables investigated accounted for a statistically significant portion (27%) of the variance in students' understanding of genetics concepts. Reasoning ability and meaningful learning orientation predicted students' understanding of genetics concepts in a positive direction, rote learning orientation predicted it in a negative direction. Additionally, the results indicated that reasoning ability explained more of the variance in test scores as compared to learning approaches.