The formation of new genetic sequences by piecing together segments of previously existing ones. Classical genetics is based on the transmission of traits, attributable to particular genetic determinants or genes, through sexual generations. The genes are predominantly present in chromosomes, the threadlike structures that fill the cell nucleus and divide as the cells divide. The key component of the chromosomes is deoxyribonucleic acid (DNA); each chromosome has a single coiled and folded double-stranded DNA molecule running along its length, and the genes of the chromosomes occupy segments of this DNA.