A geneticist needs to carry a test cross when he/she wants to determine the genotype of an organism, with a dominant phenotype trait, whether it is homozygous or heterozygous.
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Genotype of the dominant phenotype is determined by a test cross. In it F1 progeny is crossed to it recessive parents when F1 progeny (heterozygous) crossed with dwarf plant, the monohybrid test cross ratio is 1 : 1. But, all tall plants are obtained when both homozygous parents are crossed.