Genetics, the study of heredity unifies all the sub-disciplines of Bio-Sciences. Literature is replete with evidence that it is a hard-to-teach and hard-to-learn Biology topic. Advance organizers strategies had been widely reported to be effective in teaching and learning difficult topics but this needs further verification in relation to pre-instructional word clearing and mathematics pre-teaching. Therefore, the study investigated the effects of pre-instructional word-clearing and mathematics pre-teaching on senior school students’ achievement in genetics. The objective of the study were to determine the: (i) effects of pre-instructional word-clearing on students’ achievement in genetics; (ii) effects of the use of mathematics pre-teaching on students’ achievement in genetics; (iii) difference in the achievement of students based on gender when taught genetics with pre-instructional word-clearing; (iv) difference in the achievement of students based on gender when taught genetics with mathematics pre-teaching of genetics; (v) difference in students’ achievement based on study habits when taught genetics using pre-instructional word-clearing; and (vii) difference in students’ achievement based on study habits when taught genetics using mathematics pre-teaching.rnThe study adopted a quasi-experimental design involving pre-test, post-test control groups with a 3 x 2 x 2 x 3 matrix. The target population was all senior school III students offering Biology in Ilorin metropolis. Purposive sampling technique was employed to select three co-educational schools. The sample consisted of 172 senior school III students from three intact classes. The instruments used were the Genetics Achievement Test (GAT) and an adapted Questionnaire on Biology Students’ Study Habits (QBSSH). The reliability coefficient of GAT and QBSSH were 0.98 and 0.83 respectively. Hypotheses were tested using the t-test and Analysis of Covariance (ANCOVA) at 0.05 level of significance.rnThe findings of the study were that:rni. a significant difference exists in the achievement of students taught genetics using pre-instructional word-clearing and those taught with conventional strategy in favour of those taught with pre-instructional word-clearing (t(118)=3.38; p<0.05);rnii. a significant difference exists in the achievement of students taught genetics using mathematics pre-teaching and those taught with the conventional strategy in favour of those taught with mathematics pre-teaching (t(120)=-0.99; p<0.05);rniii. there was no significant difference in the achievement of male and female students taught genetics using pre-instructional word-clearing;rniv. there was no significant difference in the achievement of male and female students taught genetics using mathematics pre-teaching;rnv. there was a significant difference in the achievement of students with positive and negative study habits when taught genetics using pre-instructional word-clearing in favour of students with positive study habits (t(50) = -10.96; p < 0.05); andrnvi. a significant difference exists in the achievement of students with positive and negative study habits when taught genetics using mathematics pre-teaching in favour of students with positive study habits (t(48)= -8.98; p < 0.05).rnThe study concluded that the use of pre-instructional word-clearing and mathematics pre-teaching enhanced students’ achievement in genetics. This implies that these strategies enhance meaningful learning of difficult concepts in Genetics. The study recommended that Biology students and teachers should clarify the genetics terminologies and review the mathematics concepts related to genetics concepts prior to instruction to boost students’ Achievement.