The F₂ cross  is the quintessential segregating generation.  For a single autosomal gene,  the F₂ generation is the first generation where both parental phenotypes occur.  For two or more genes, it is the first generation where all new combinations of phenotypes can occur at the same time.  These new phenotypes reflect independent assortment among the alleles genes that formed the gametes of the F₁ generation.

As previously discussed,  there are two possible F₁ generations called the F₁ and the F₁' (or F₁ reciprocal).  Thus, there are four possible F₂  generations (see below for derivation).  For autosomal traits, these four F₂ generations are the same.  For sex-linked and Y-linked traits, these four generations are different. (for the ZW system click here (inactive link)  

In order to calculate the expected phenotype and the phenotypic frequencies it is essential to know the dominance patterns and whether the trait is autosomal or sex-linked.  This information is obtained from the observed F₁ data. (for review of F₁ crosses click here).  For two or more loci, the phenotypic ratios may be modified by linkage.