Eukaryotic cells contain two versions of genes, referred to as alleles. Alleles pair with one another during meiosis, which is the process of nuclear cell division during reproduction.
A human being receives one allele from each parent. Alleles correspond to different traits, such as left-handedness or blue eyes. During the process of meiosis, chromosomes line up in pairs, or alleles, and then wrap themselves around each other randomly. This event causes a diversification in traits, which causes each set of chromosome pairs to be uniquely ordered when replicated. This process is known as crossing over, and at the end of it, alleles are first separated and then paired anew to form gametes, which are male or female sex cells. When the two new alleles that come together are the same, that set of alleles is known as homozygous, which indicates a high likelihood that the resulting offspring will possess that particular trait. When the allele pairs are different, they are referred to as heterozygous, which means there is a 50 percent chance that the offspring will possess the particular trait that corresponds to that allele. This type of genetic recombination is essential to the creation of a person's individual DNA.