Test Cross: Determining Genotype — NEET Biology
✅ Asked in NEET 2025From the preceeding paragraphs it is clear that though the genotypic ratios can be calculated using mathematical probability, by simply looking at the phenotype of a dominant trait, it is not possible to know the genotypic composition. That is, for example, whether a tall plant from F1 or F2 has TT or Tt composition, cannot be predicted. Therefore, to determine the genotype of a tall plant at F2, Mendel crossed the tall plant from F2 with a dwarf plant. This he called a test cross. In a typical test cross an organism (pea plants here) showing a dominant phenotype (and whose genotype is to be determined) is crossed with the recessive parent instead of self-crossing. The progenies of such a cross can easily be analysed to predict the genotype of the test organism. shows the results of typical test cross where violet colour flower (V) is dominant over white colour flower (v).
A test cross is used to determine the unknown genotype of an organism displaying a dominant phenotype by crossing it with a recessive homozygote. The key concept NTA tests is: you cannot determine if a dominant plant is homozygous (TT) or heterozygous (Tt) just by looking at it—you must perform this cross. Students commonly mistake thinking that all dominant phenotypes are homozygous, leading to wrong genotypic predictions. Remember: a 1:1 phenotypic ratio in offspring indicates the test organism is heterozygous (Tt), while all dominant offspring indicate homozygosity (TT). This appeared in NEET 2022 and 2025, making it a high-yield concept for prediction questions.
This paragraph was tested 2 times in NEET.
A plant with purple flowers (genotype unknown) was crossed with a white-flowered plant (ww). In the progeny, half of the flowers were white. What is the genotype of the purple-flowered parent? (NEET 2025)
A heterozygous pea plant with violet flowers was crossed with a homozygous pea plant with white flowers. Violet is dominant over white. Which one of the following represents the expected combinations among 40 progenies formed? (NEET 2022)
Through deep analysis of NEET and NTA, 88 of 90 questions from the NEET 2026 paper were matched straight from the MedicNEET Biology question bank.
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