Gamete Types in Heterozygous Organisms — NEET Biology

Correct answer: B — S1, S3 and S4 are correct

This question focuses on the test cross and the Law of Segregation. S1: A monohybrid test cross involves crossing an organism showing a dominant phenotype (whose genotype is unknown) with the homozygous recessive parent. This is its definition and purpose. So, S1 is correct. S2: The progeny of a monohybrid test cross (e.g., Tt x tt) will show a phenotypic ratio of 1:1 (e.g., tall:dwarf). A 3:1 phenotypic ratio is characteristic of an F2 generation from a monohybrid cross (Tt x Tt). So, S2 is NOT correct. S3: According to the Law of Segregation, alleles do not blend and separate during gamete formation, such that each gamete receives only one allele from the pair. A heterozygous parent (Tt) produces two types of gametes (T and t) in equal proportions (50% T, 50% t). So, S3 is correct. S4: If a test cross results in all progeny showing the dominant phenotype, it indicates that the organism with the dominant phenotype was homozygous dominant (e.g., TT × tt -> all Tt tall). So, S4 is correct. S5: The Law of Dominance explains the expression of only one parental character in F1. The Law of Segregation ensures that alleles separate during gamete formation, which directly underlies the 1:1 ratio in a monohybrid test cross. The Law of Independent Assortment applies to dihybrid crosses, stating that different pairs of traits segregate independently. It is not the primary law explaining the 1:1 ratio in a monohybrid test cross which is a direct consequence of segregation. So, S5 is NOT correct. Therefore, S1, S3 and S4 are correct.