According to the sources, the ABO grouping is determined by the presence or absence of two surface antigens on the Red Blood Cells (RBCs), specifically antigens A and B . Individuals with blood group O are characterised by having no (nil) antigens on the surface of their RBCs . Genetically, this is controlled by the gene I, where the recessive allele $i$ does not produce any sugar polymers (antigens), whereas the alleles $I^A$ and $I^B$ produce slightly different forms of sugar . Therefore, an individual with the genotype $ii$ results in the O blood type . Understanding these fundamental physiological and genetic principles is a high-priority strategy for medical aspirants, as NCERT textbooks serve as the primary source for approximately 95% of NEET Biology questions .