Q.Explain the following terms with example

Let’s take these one at a time, exactly as the NCERT textbook presents them.

(a) Co-dominance

In co-dominance, both alleles of a gene express themselves fully and equally in the heterozygote — neither is dominant or recessive. The offspring shows both parental traits side by side, not a blend.

• NCERT example: ABO blood group system in humans. The allele I^A and I^B are co-dominant. A person with genotype I^A I^B has both A and B antigens on red blood cells, producing blood group AB. Both traits appear together.

(b) Incomplete dominance

In incomplete dominance, the heterozygote shows an intermediate phenotype — a blend — because neither allele is completely dominant over the other. …

To understand these two terms properly, you must first recall the core of Mendelian genetics. When Mendel crossed a pure tall pea plant with a pure dwarf pea plant, all the F1 offspring were tall. The tall allele completely masked the dwarf allele. That is complete dominance — one allele dominates over the other, and the heterozygote looks exactly like one of the homozygotes.

But nature is not always that tidy. Sometimes, the heterozygote does not resemble either parent. That is where co-dominance and incomplete dominance enter the picture. They are both cases where the F1 hybrid shows a new phenotype, but the mechanism behind that new appearance is different in each case.

(a) Co-dominance

In co-dominance, both alleles in a heterozygote express themselves fully and independently. Neither allele is dominant or recessive. The heterozygote shows the traits of both parents simultaneously, not a blend.

The NCERT textbook gives the classic example: ABO blood groups in humans. The gene I (for isoagglutinin) has three alleles: I^A, I^B, and i. I^A and I^B are co-dominant with each other, while both are dominant over i.

Consider a person with genotype I^A I^B. This person has both A antigens and B antigens on the surface of their red blood cells. The blood group is AB. The person is not "half A and half B" in some blended sense — they are fully A and fully B at the same time. Both alleles are actively producing their respective antigens. That is co-dominance.

Another example from NCERT: coat colour in cattle (specifically the Roan breed). When a red-haired cow is crossed with a white-haired bull, the offspring (Roan) has a coat with intermingled red and white hairs. Each hair is either fully red or fully white — there is no "pink" hair. The two colours appear together, not blended. That is co-dominance at the cellular level.

(b) Incomplete dominance

In incomplete dominance, neither allele is fully dominant over the other. The heterozygote shows a phenotype that is intermediate between the two homozygous parents — a blending effect.

The NCERT textbook uses the famous example from snapdragon (Antirrhinum majus), also called dog flower. When a pure red-flowered snapdragon (RR) is crossed with a pure white-flowered snapdragon (rr), the F1 generation has pink flowers (Rr). The pink colour is intermediate between red and white.

Now, here is the crucial part: if you self-pollinate these pink F1 plants, the F2 generation shows a phenotypic ratio of 1 Red : 2 Pink : 1 White. Notice that the red and white parental types reappear in the F2. This proves that the alleles have not "mixed" permanently — they remain separate and segregate according to Mendel's law. The pink colour is not a new allele; it is the result of the red allele producing only half the normal amount of red pigment, so the flower appears lighter. …

A quick way to tell the two apart under exam pressure: co-dominance is like flying two separate flags side-by-side on the same pole (both colours fully visible, e.g. the AB blood group showing both A and B antigens), while incomplete dominance is like mixing two paints into one new colour (a blend, e.g. red and white parents giving pink flower …