Mendelian inheritance and probability
In an Aa × Aa cross with complete dominance, expected genotypes are 1 AA : 2 Aa : 1 aa and phenotypes are 3 dominant : 1 recessive. These are probabilities over many offspring, not a quota for every family. A testcross pairs an individual showing a dominant phenotype with a homozygous recessive individual. Independent assortment of two loci supports multiplying their separate probabilities; linkage can invalidate that shortcut. Conditional questions change the group being considered.
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Among offspring of Aa × Aa that show the dominant phenotype, 2/3 are expected to be Aa because the aa offspring have been excluded.
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Dominant does not mean common, stronger, or selectively favored. It describes a heterozygote phenotype.