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Combination of alleles, found at the same locus, that are both identical and will produce only the associated phenotype upon test cross.
If we consider the genotype to be AA, where A is the dominant allele, then this individual would be a homozygote. Further, in the test cross AA × aa, all progeny would be Aa. Since A is dominant to a, by definition, i.e., as considered here, then the phenotype associated with Aa would be the same as the phenotype associated with AA.
Whenever all progeny possess the same phenotype following a test cross then you an be pretty sure that the unknown genotype is homozygous. After that it is simply a matter of determining whether the phenotype observed among these progeny is dominant versus recessive, something that may be intuited but more generally simply requires knowledge of the genetics of character in question.
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