Which phenotypic ratio is likely to occur in a dihybrid cross of two completely dominant, independently segregating gene pairs when both parents are fully heterozygous?

9:3:3:1

Explanation:

If two parents for a dihybrid cross are fully heterozygous for two genes, the phenotype ratio comes 9:3:3:1. Here, the alleles of both genes should exhibit complete dominance-recessiveness. This means that one allele of a gene should be able to mask the expression of its another allele. Similarly, alleles of one gene genes should assort in a random manner during anaphase-I irrespective of the assortment of alleles of another gene.

Under these conditions, each parent produces four types of gametes in equal proportion. And the random fusion of these gametes gives progeny in 9:3:3:1 phenotype ratio.  

For example, a dihybrid cross between TtRr and TtRr would produce progeny in the following phenotype ratio=

9 tall and red: 3 tall and white: 3 dwarf and red: 1 dwarf and white

Here the allele T gives "tallness" while the allele "t" gives "dwarfism". The dominant allele "R" gives "red flowers" while the recessive allele "r" gives white flowers.