Test Cross Definition and Purpose — NEET Biology
📚 Practice ConceptUsing the genotypes of this cross, can you give a general definition for a test cross?
Which of the following statements are correct regarding the F₂ generation of a dihybrid cross involving two independently assorting genes with complete dominance? S1: In the F2 generation of a dihybrid cross (e.g., RrYy x RrYy), there are 9 different genotypes. S2: The genotype Rryy appears with a frequency of 1/8 in the F2 generation. S3: The probability of obtaining an offspring with at least one dominant allele for both traits (R_Y_) in the F2 generation is 9/16. S4: The genotypic ratio of the F2 generation can be expressed as a product of two independent monohybrid genotypic ratios (1:2:1) * (1:2:1). S5: The genotype rrYY appears twice as frequently as RRYY in the F2 generation.
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Correct answer: D — S1, S2, S3 and S4
This question focuses on the genotypic and phenotypic outcomes of the F2 generation in a dihybrid cross assuming independent assortment and complete dominance. S1: From the Punnett square for a dihybrid cross (RrYy x RrYy), there are 9 distinct genotypes (RRYY, RRYy, RRyy, RrYY, RrYy, Rryy, rrYY, rrYy, rryy). So, S1 is correct. S2: The genotype Rryy appears 2 times out of the 16 possible F2 combinations. Thus, its frequency is 2/16, which simplifies to 1/8. So, S2 is correct. S3: The phenotypic class with at least one dominant allele for both traits (Round Yellow, R_Y_) corresponds to the '9' portion of the 9:3:3:1 phenotypic ratio. Therefore, the probability is 9/16. So, S3 is correct. S4: The F2 genotypic ratio for a monohybrid cross is 1:2:1. For a dihybrid cross with independent assortment, this can be mathematically derived as the product of two monohybrid genotypic ratios: (1:2:1) x (1:2:1). So, S4 is correct. S5: In the F2 generation, the genotype rrYY appears once (1/16), and the genotype RRYY also appears once (1/16). Thus, rrYY appears with the same frequency as RRYY, not twice as frequently. So, S5 is incorrect. Therefore, statements S1, S2, S3, and S4 are correct.
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Through deep analysis of NEET and NTA, 176 of the 180 ReNEET 2026 (June 21) questions were already in the MedicNEET question bank before the exam.
176/180MedicNEET's Biology question bank is built from the same NCERT lines NTA picks repeatedly. Not random MCQs — questions crafted exactly like NTA crafts them.