As explained in Section 1.3, each sperm cell and each egg cell is genetically unique. Every one of us arose from a single cell that formed by the fusion of a unique sperm and a unique egg cell.
Genetic variation describes differences between the DNA sequences of individual genomes. Because each of us has two nuclear genomes (a paternal genome and a maternal genome), genetic variation occurs within as well as between individuals. At any genetic locus (DNA region having a unique chromosomal location) the maternal and paternal alleles normally have identical or slightly different DNA sequences (we are said to be homozygotes if the alleles are identical, or heterozygotes if they differ by even a single nucleotide).