When two pure organisms are crossed, the first generation of offspring will be all heterozygous and will show the dominant phenotype.

Let's represent the two pure organisms as AA and aa, where A is the dominant allele and a is the recessive allele.

* P generation: AA x aa

* F1 generation: All Aa (heterozygous dominant)

If the F1 generation is self-crossed, the second generation of offspring (F2 generation) will show a 3:1 ratio of dominant to recessive phenotypes.

* F2 generation:

- 3/4 Aa (heterozygous dominant)

- 1/4 aa (recessive homozygous)

This is known as Mendel's law of segregation. It states that the alleles for a gene separate (segregate) during gamete formation and recombine randomly during fertilization.

The F2 generation will also include a new genotype: the homozygous recessive aa. This genotype will only be produced if both parents are heterozygous Aa.

The genotypic ratio of the F2 generation is 1 AA : 2 Aa : 1 aa. This ratio can be expanded to show the phenotypic ratio of 3 dominant: 1 recessive.

The law of segregation applies to all genes, not just those that show simple dominance and recessiveness. It is one of the fundamental laws of genetics and helps to explain how traits are inherited from parents to offspring.