Biology · Principles of Inheritance and Variation · NEET
In incomplete dominance, the heterozygote shows a NEW blended trait that is in-between the two parents (red + white = pink). Neither allele is fully seen; they mix. In codominance, the heterozygote shows BOTH traits fully and separately at the same time, with no blending (blood group AB has both A antigen and B antigen). Trick to remember: incomplete = blend (one new look), codominance = both (two looks together).
AB blood group is CODOMINANCE, not incomplete dominance. This is a very common NEET trap. In a person with genotype I^A I^B, the I^A allele makes A-type sugar and the I^B allele makes B-type sugar, and BOTH sugars appear fully on the red blood cells. There is no blending into a 'mixed' sugar. NCERT states clearly: 'when I^A and I^B are present together they both express their own types of sugars: this is because of co-dominance.'
Because the heterozygote (Rr) is a single NEW colour, pink, which is a blend of red and white. You do not see red patches AND white patches on the same flower. If both colours appeared fully and separately, THAT would be codominance. Since red and white mix into one in-between colour (pink), it is incomplete dominance. The gene R is simply not completely dominant over r.
Yes, both still follow the Law of Segregation. This confuses many students. The alleles still separate cleanly into gametes and pair up again. In snapdragon, the F2 genotype ratio is exactly 1 RR : 2 Rr : 1 rr, just like any normal monohybrid cross. What changes is only the Law of DOMINANCE, because the heterozygote does not look like one pure parent. The genotype ratio stays perfectly Mendelian.
In incomplete dominance, the F2 phenotypic ratio is 1 : 2 : 1 (for snapdragon, 1 Red : 2 Pink : 1 White). This is the SAME as the genotype ratio because every genotype gives a different look. Codominance (like ABO) also gives 1 : 2 : 1 style genotype-matches-phenotype patterns because the heterozygote is distinct. So the key change in BOTH is: the 3 : 1 ratio of complete dominance becomes 1 : 2 : 1, because the heterozygote is now visible as its own class.
In both cases, neither allele is fully dominant or fully recessive over the other, so both alleles show some effect in the heterozygote. The difference is HOW: in incomplete dominance they blend into one middle trait; in codominance they each show fully and separately. This is why the heterozygote is always distinguishable from both parents in both cases, which is why the 1:2:1 ratio appears.
Match List I with List II: List-I: A. Incomplete dominance B. Co-dominance C. Pleiotropy D. Polygenic inheritance List-II: I. Human skin colour II. Inheritance of flower colour in Antirrhinum sp. III. Phenylketonuria disease in humans IV. ABO blood groups
Identify the wrong statement with reference to the gene 'I' that controls ABO blood groups.
In Antirrhinum (Snapdragon), a red flower was crossed with a white flower and in F1 generation pink flowers were obtained. When pink flowers were selfed, the F2 generation showed white, red and pink flowers. Choose the incorrect statement from the following:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Different alleles of the same gene can behave differently. In the ABO gene, I^A and I^B are codominant with each other, but each is completely dominant over i. So within one gene you can see complete dominance (I^A over i) and codominance (I^A with I^B). Incomplete dominance is a separate case seen in genes like snapdragon flower colour.
No. In codominance the F1 shows BOTH parent traits fully and separately, not a mix. NCERT says in codominance 'the F1 generation resembles both parents.' In incomplete dominance the F1 is an in-between blend that resembles NEITHER parent exactly (like pink). This resemblance rule is a fast way to tell them apart.
In complete dominance, the heterozygote looks the same as one homozygote, so two genotypes share one look, giving 3:1. In incomplete dominance the heterozygote has its own unique look (pink), so each of the three genotypes RR, Rr, rr is visible separately. This makes the phenotype ratio 1:2:1, matching the genotype ratio.
Both, but the EXAMPLES win marks most often. NEET repeatedly tests 'Antirrhinum/snapdragon = incomplete dominance' and 'ABO blood group = codominance' in match-the-column and statement questions. Memorise these two example-to-term links exactly, and never swap them.
