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The Test Cross Calculator predicts offspring ratios when crossing an organism with a dominant phenotype (unknown genotype) to a homozygous recessive (aa) tester. A test cross is the classic genetic technique for determining whether an individual showing the dominant phenotype is homozygous (AA) or heterozygous (Aa).
Enter whether the unknown parent is AA (2 dominant alleles) or Aa (1 dominant allele) and the total offspring count to see expected results for each scenario.
The test cross works by crossing the unknown individual with a homozygous recessive (aa):
If AA × aa: All offspring are Aa (100% dominant phenotype). No recessive offspring appear.
If Aa × aa: Offspring are 50% Aa (dominant) and 50% aa (recessive). The 1:1 ratio reveals heterozygosity.
The key insight is that the homozygous recessive tester can only contribute recessive alleles, so any recessive offspring must have received a recessive allele from the unknown parent, proving it is heterozygous.
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A heterozygous parent crossed with aa tester produces a 1:1 ratio: 50 dominant and 50 recessive offspring expected from 100 total.
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A homozygous dominant parent crossed with aa produces 100% dominant offspring. All 100 offspring show the dominant phenotype.
The test cross is the definitive way to determine an unknown genotype of a dominant-phenotype individual. Since AA and Aa look identical, you cannot tell them apart by phenotype alone. By crossing with a homozygous recessive, any recessive allele in the unknown parent will be revealed in the offspring.
If any recessive offspring appear, the unknown parent must be heterozygous. If all offspring are dominant, the parent is likely homozygous, but certainty increases with sample size. With 7 offspring all showing dominant phenotype, there is a 99.2% confidence that the parent is AA (since the chance of 7 consecutive dominant from Aa × aa is 0.5⁷ = 0.78%).
Yes, test crosses work for any organism and any gene with complete dominance. In plants, you cross the unknown with a homozygous recessive plant. In animals like Drosophila, you cross with a recessive-phenotype fly. The principle is universal across all sexually reproducing organisms.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
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