Final answer:
To determine dominant alleles, a test cross is used by crossing an organism with a dominant phenotype with a homozygous recessive organism, which reveals the genotype of the dominant organism based on the offspring phenotypes.
Step-by-step explanation:
Determining Dominant Alleles
To determine which of two alleles of a gene is dominant, a test cross is the most effective method. A test cross involves crossing an organism that expresses the dominant phenotype with one that is homozygous recessive for the same gene. If the organism with the dominant phenotype is homozygous dominant (VV), all F1 offspring will express the dominant trait. However, if the organism is heterozygous (Vv), there will be a 1:1 ratio of dominant to recessive phenotypes among the offspring. This technique validates Mendel's principle of equal segregation of alleles.