Mendel's first law, called the law of segregation, states the idea that alleles segregate, or separate, when the F1 generation produces gametes. Alleles reside at specific loci, or sites, on the chromosomes within the DNA molecule. A pair of homologous chromosomes contains two alleles at each locus and during gamete formation of meiosis, each gamete receives only one member of each homologous pair of chromosomes. Therefore, each gamete also recieves only one allele of a particular locus.
Take, for example, the crossing of two homozygous parents of particular flower: one having the color purple, represented by PP, and the other having the color red, or pp. When these two are crossed, all the offspring, or the F1 generation, are purple - Pp. When this generation is self crossed, the P and p alleles separate to form three different possible genotypes - PP, Pp (and pP), and pp. These genotypes make up the F2 generation. Three fourths of the gametes have the phenotype of purple and one fourth have the phenotype red. The Punnett Square method can be used to figure out these ratios.
This image was created by Pratima Agrawal.