Mendel’s law of segregation states that during the gametes production the two copies of each hereditary factor separate so that progeny receive one factor from each parent.
Observing that breeding pea plant with different traits gave rise to F1 generations in which all the plants expressed the dominant trait and F2 generation in which some plants expressed the recessive and dominant traits in the ratio of 1:3, Mendel proposed the law of segregation. According to that law, each individual that is diploid have a pair of alleles for a particular trait. At random, each parent passes an allele to their progeny resulting in a diploid organism. The allele with the dominant trait determines the offspring phenotype. In essence, the law of segregation states that copies of gene segregate or separate so that each gamete gets only one allele.
The physical basis of the law of segregation is the 1st division of meiosis where the homologous chromosomes with versions of each gene are separated into daughter nucleic. The homologous chromosomes behaviour during meiosis accounts for the separation of the alleles to different gametes. As the chromosome separate into different gametes, the two alleles for a particular gene also separate so that each gamete receives one of the two alleles.