Gregor Mendel's first principle of genetic inheritance. It states that, for any particular trait, the pair of genes of each parent separate (during the formation of sex cells) and only one gene from each parent passes on to an offspring. In other words, genes occur in pairs (because chromosomes occur in pairs). During gamete production, the members of each gene pair separate, so that each gamete contains one member of each pair. During fertilization, the full number of chromosomes is restored, and members of gene pairs are reunited.