The number of possible alleles and genotypes at a QTL in outbred linecross designs (backcross and F2 intercross with and without assumed fixation of QTL alleles in different lines)

Outbred (line) crossBackcrossF2
Fixed grandparental lines have been assumed2 alleles2 alleles
2 genotypes3 genotypes
General (no assumed fixation)3 alleles4 alleles
4 genotypes4 genotypes

Here genotype AB is considered to be the same as BA. F2 without assumed fixation corresponds to a full-sib family of outcrossing (cross-pollinating) species.