The study explores students' use of language in the process of making sense of genetics concepts. It aims to analyze primary and secondary discourses, and examine the relationship between social practices and discourses. Sixth-grade students were interviewed before and during four weeks instruction on genetics. General trends were detected regarding border crossing between discourse communities and the difficulties and ease of moving among informal experience, social practices, primary discourse of family, friends and community, and secondary discourse of science and school instruction. Two comprehensive case summaries--Debbie and Sam--are presented in the paper. Debbie took what she experienced in her social life as criteria to discuss how people resembled one another. However, the language practiced in the classroom was different from Debbie's familiar language. Debbie thus experienced a conflict between primary and secondary discourses. Sam's school education and home environment provided an aligned social context with rich scientific ties that nurtured his use of secondary discourse practices in thinking and learning science. We recommend that science instruction needs to build a learning community where students' discourses will be recognized and border crossings between discourses facilitated in the process of learning science.