BRAINCAKETest Cross
An organism showing a dominant phenotype could be homozygous dominant (TT) or heterozygous (Tt), because both look the same. A test cross is used to find out which.
In a test cross, the organism with the dominant phenotype is crossed with an organism that is homozygous recessive (tt) for the same gene. The homozygous recessive parent can only make t gametes, so the phenotype of each offspring reveals which allele it received from the unknown parent.
If the unknown parent is TT:
TT × tt → gametes T and t → all offspring Tt
All offspring show the dominant phenotype.
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