This study aimed to provide a deeper understanding of high school students' errors while solving genetics problems. Four students were asked to solve two genetics problems and participate in a modified think-aloud interview. Data from the interview, video clips of the problem-solving process, and test sheets were used to analyse participants' problem-solving processes and identify errors during each of the following problem-solving phases: understanding, planning, execution, and checking. The error of "Misinterpreting evidence" was observed in the understanding phase for Problem 1 (represented verbally), whereas the error of "Making incorrect assumptions" was observed for Problem 2 (students had to draw a pedigree). Errors of "Incorrect conclusion" occurred in the planning phase, characterised by problem-solving strategies based on incorrect assumptions or misunderstanding the problem due to errors in the understanding phase. Further, "Using incomplete representation" and "Mathematical errors" were observed in the execution phase due to representational competence and the application of mathematical principles, respectively. The checking phase was characterised by "Not monitoring the problem-solving process" and "Disregarding evidence," wherein participants failed to check the logical consistency of genetic principles with the information embedded in the problem. The results indicated that problem-solving errors were related to the interaction between the students' domain-general competencies and domain-specific knowledge.