The explanation for heterosis phenomenon is based on ideas: (i) every trait of an organism depends on many genes. (ii) Inbreeding depression and heterosis are related to individual genetic diversity. To assess individual genetic diversity of an organism, I suggest the term number of genetic properties. Assessing the changes of individual genetic diversity caused by self-pollination and cross-pollination reveals that self-pollinating plants of natural cross-pollinating leads to the decrease in individual genetic diversity of offspring and crossing between pure lines of genetic difference leads to the increase in individual genetic diversity of hybrids. Therefore, I propose that the decrease in individual genetic diversity is the cause the of depression and the increase in individual genetic diversity is the cause of heterosis.