By analyzing the preconceptions of sixth grade pupils, there were determined some preconceptions and misconceptions, and by verifying it with a written exam done by pupils of the eighth grade and by qualitative and quantitative analysis of the results the amount learned and conceptual understanding of concepts such as mitosis, mejosis and inheritance was checked with pupils of elementary school. In the research done the participants were pupils of sixth and eighth grade from Croatian schools in June 2011. The results show that a large number of pupils have difficult in connecting these concepts and that failure prevails, while only the most successful pupils accomplish conceptual understanding. Preconceptions determined as a good foundation in the future teachings is knowing the genes and/or DNA as a carrier of genetic information, the uniqueness of a DNA molecule, the existence of DNA analysis. The miscenceptions that were giving the pupils and the teachers trouble with learning and teaching have been checked and are as follows: small part of the pupils understands what makes a DNA molecule and what she is a part of, they mix the number of cells with the number of chromosomes in cellular division, they don't understand the relationship of the elements that make the transference of inheritance instructions happen, they can't tell the difference between animal and plant cells, they connect Y chromosome with an egg cell, and they don't understand the synthesis of proteins.