If all F2 progeny show dominant phenotype, the genotype of F1 was
If all F2 progeny exhibit dominant phenotype after selfing, parental F1 could not have contributed recessive allele to any zygote, genotype must be homozygous dominant TT producing only T gametes. Heterozygous Tt would have generated 25 percent recessive tt progeny giving 3:1 ratio, tt would produce all recessive, mixture Tt and TT would also produce some recessive when Tt mothers contribute t. Uniformly dominant F2 therefore indicates fixation of dominant allele in F1, genotype TT only capable of such outcome. Conceptual clarity supports solving numerical problems involving segregation ratios
Ref: Hartl & Ruvolo, Genetics, 9th ed., Chapter 2: Homozygous Dominant F1 Identification