Even in the 21st century, textbooks can play a significant role in fostering higher-order thinking skills (HOTs) for effective learning and understanding. Still, HOTs like analogies used in biology textbooks may miss their purpose in promoting in-depth understanding. Since most analogies are presented implicitly or partially explicitly, they lack an explicit mapping to explain the analogical pattern between the source and the target. This study examines the degree to which implicit and explicit expressions of four Relational Reasoning skills (RRs): Analogy, Antinomy, Anomaly, and Antithesis, appear in three biology textbooks taught in Israel's junior high school. Qualitative content analysis crosses four predetermined criteria (C1-C4): RRs' type, texts' type (T1, T2, T3), 'mapping process', and 'use-skill indication'. The quantification of the findings provided information on the RRs' distribution and prevalence. The study's findings indicate that only 14% of texts appear with explicit expressions for RRs, mainly in antinomies questions (T2). Although about 32% of the various texts in biology include instructions for learners to activate HOT by using RRs, they are presented at a partially explicit level. Moreover, less than 2% of activities (T3) explicitly enable HOT by using RRs to solve problems. This study expands the theoretical knowledge of analogies to all four RRs. Methodologically, the study presents explicit mapping processes developed for antinomies, anomalies, and antitheses. The implications of the RRs' degree of explicitness are discussed as an effective index of learners' scientific understanding