Segregation of alleles refers to Mendel's Law of Segregation. Mendel's Law of Segregation states that allele pairs separate during gamete formation and reform randomly during fertilization.
All diploid organisms have two copies of every gene within their DNA. These gene copies are referred to as alleles, and they may differ slightly from one another in their sequences. According to Mendelian genetics, the combination of these alleles, also known as the genotype, is what gives an organism its phenotypic, or outward traits.
Each pair of alleles from each parent separates during formation of egg and sperm cells. This process, called meiosis, results in the creation of haploid egg and sperm cells, which contain only one copy of each allele.
During fertilization, haploid egg and sperm cells combine at random, forming new diploid cells with two alleles for each gene, one from each parent. Since this is random, the chance of getting a particular allele from one parent is 50 percent. Therefore, one organism inherits two randomly paired alleles, or one allele from each parent, per trait.
The final phenotype that is displayed in the organism depends on which allele is dominant. For example, if brown eye color is a dominant trait, indicated with a capital "B", and blue eye color is a recessive trait, indicated with a lower case "b", an organism with one copy of each allele, or genotype "Bb", should have brown eyes. The egg or sperm cells of this organism are compromised of 50 percent "B" cells and 50 percent "b" cells. The phenotype of the offspring depends on the genotype of the particular organism's mating partner.