Key Genetic Terminology for Inheritance — NEET Biology

Correct answer: D — S3, S4 and S5 are not correct

The core concept revolves around the Law of Dominance, and distinguishing phenotypes from genotypes. S1: The Law of Dominance states that in a dissimilar pair of factors (alleles), one member dominates the other. This explains why only one parental character is expressed in F1 and both are expressed in F2 in a 3:1 ratio. So, S1 is correct. S2: In a cross between true-breeding tall (TT) and dwarf (tt) pea plants, the F1 generation will all be tall (Tt), resembling one parent. This is due to dominance. So, S2 is correct. S3: Homozygous dominant (TT) and heterozygous (Tt) genotypes can be phenotypically identical (e.g., both tall). Therefore, a test cross is used to distinguish them, by crossing with the homozygous recessive (tt). The statement says they are always distinguishable phenotypically, which is incorrect as they can appear the same. So, S3 is NOT correct. S4: The reason a recessive trait is expressed is typically due to a non-functional enzyme or no enzyme production by the recessive allele. The statement claims it's due to a more efficient enzyme from the recessive allele, which reverses the concept of recessiveness at the molecular level. So, S4 is NOT correct. S5: Test cross progeny show a phenotypic ratio of 1:1 for a monohybrid cross if the dominant parent is heterozygous. If the dominant parent is homozygous dominant, the ratio would be 1:0. The statement claims it always yields a 3:1 ratio if the parent is heterozygous, which is incorrect. The 3:1 ratio is for F2 self-pollination. So, S5 is NOT correct. Therefore, S3, S4, and S5 are NOT correct.