Class XII Biology • NCERT Based • NEET Practice
The wrong statement is option 3. In the ABO system, IA and IB are codominant; when both are present (IAIB), the person has AB blood type with both A and B antigens. The statement says they express the same sugar, which is incorrect because A and B antigens are different sugars on RBC surfaces.
🧠 Did You Know?Allele i is recessive and does not code for A or B antigens; individuals with ii blood type have no A or B antigens (O blood group).
💡 NCERT Memory Line:The gene I has three alleles (IA, IB, i); a person carries two alleles, and IA and IB show codominance to yield AB type when present together.
Option 3 is correct. Sickle cell anaemia is an autosomal recessive disorder caused by a mutation in the beta-globin gene on chromosome 11. Children must inherit two faulty copies to express the disease. Options 1, 2 and 4 are incorrect according to NCERT: haemophilia is X-linked recessive (not Y-linked); phenylketonuria is autosomal recessive (not dominant); thalassemia is also autosomal recessive (not X-linked).
🧠 Did You Know?Carriers (heterozygotes) for sickle cell trait usually do not show full disease but may have some abnormal Hb; this trait is more common in populations where malaria is or was prevalent.
💡 NCERT Memory Line:“Sickle cell anaemia is an autosomal recessive disorder due to mutation in beta-globin gene on chromosome 11.”
The best example of pleiotropy is Phenylketonuria (PKU). In pleiotropy, a single gene affects many traits. The PKU gene causes a deficiency of the enzyme phenylalanine hydroxylase, leading to accumulation of phenylalanine and affecting multiple body processes. This results in various problems such as intellectual disability, growth issues, and other metabolic effects, illustrating one gene influencing several traits.
🧠 Did You Know?NCERT discusses pleiotropy with examples like PKU to show how a single gene mutation can impact more than one phenotype.
💡 NCERT Memory Line:Pleiotropy: one gene, many effects.
The cross red x white in Antirrhinum shows incomplete dominance, yielding pink in F1. In F2, segregation of alleles gives 1 red : 2 pink : 1 white, confirming incomplete dominance and that the Law of Segregation does apply (gametes carry alleles, and they segregate during formation). Therefore statement 1 is incorrect. Statements 2, 3, and 4 are consistent with NCERT: ratio 1/4 red, 1/2 pink, 1/4 white; pink in F1 results from incomplete dominance; and the experiment does not follow complete dominance.
🧠 Did You Know?In incomplete dominance, heterozygotes show a third phenotype (pink) distinct from either homozygote (red or white).
💡 NCERT Memory Line:Antirrhinum shows incomplete dominance; F2 phenotype ratio is 1:2:1 (red:pink:white) due to segregation of alleles.