Incomplete Dominance

NCERT grounding

The NCERT Class 12 chapter Principles of Inheritance and Variation places incomplete dominance immediately after the Law of Segregation, as Section 4.2.2.1. The text states plainly that when experiments on peas were repeated using other traits in other plants, it was found that sometimes the F1 had a phenotype that did not resemble either of the two parents and was in between the two. The chosen illustration is the inheritance of flower colour in the dog flower — snapdragon, or Antirrhinum sp.

In a cross between a true-breeding red-flowered plant (RR) and a true-breeding white-flowered plant (rr), the F1 (Rr) is pink. When this F1 is self-pollinated, the F2 appears in the ratio 1 (RR) red : 2 (Rr) pink : 1 (rr) white. NCERT notes that the genotype ratios were exactly as expected in any Mendelian monohybrid cross, but the phenotype ratios had changed from the 3:1 dominant : recessive ratio. The NIOS supplement reinforces the same pattern with a second plant, the four o'clock plant Mirabilis jalapa, and records that its F2 phenotypic ratio is 1 red : 2 pink : 1 white.

"What happened was that R was not completely dominant over r and this made it possible to distinguish Rr as pink from RR (red) and rr (white)." — NCERT, Class 12 Biology, Section 4.2.2.1

Why the F1 is intermediate and the F2 is 1:2:1

Incomplete dominance is best defined as a pattern of inheritance in which neither member of an allele pair is fully dominant, so the heterozygote expresses a phenotype that lies between those of the two homozygotes. It is sometimes called partial dominance. The word "incomplete" is doing precise work here: the dominant allele is not absent, it simply fails to mask the recessive allele completely, leaving the heterozygote with a visibly distinct, intermediate appearance.

To set up the standard NEET cross, assign R to the red-flower allele and r to the white-flower allele in snapdragon. A true-breeding red plant is homozygous RR; a true-breeding white plant is homozygous rr. These two are the parental (P) generation. Each parent is homozygous, so each produces only one type of gamete — RR yields R gametes, rr yields r gametes. Every F1 zygote therefore receives one R and one r and is heterozygous Rr. Because R is only partially dominant, the F1 is not red; it is pink.

The intermediate phenotype is the diagnostic feature

The single most important observation in incomplete dominance is that the heterozygote has its own phenotype. In a normal monohybrid cross — for example tall (TT) crossed with dwarf (tt) — the F1 Tt is tall, indistinguishable from the TT parent, because the dominant allele fully masks the recessive one. In snapdragon, the F1 Rr looks like neither RR nor rr. It is a third, identifiable phenotype. This is precisely why incomplete dominance is classed as an exception to the Law of Dominance: NCERT's First Law says that in a dissimilar pair of factors one dominates the other, and here neither does so completely.

Tracking the F2: why segregation still gives 1 : 2 : 1 genotypes

When the pink F1 (Rr) is self-pollinated, each plant produces two kinds of gametes — R and r — in equal proportion of one-half each. This is the Law of Segregation operating exactly as it does in any monohybrid cross; incomplete dominance does not touch it. Random fertilisation of these gametes produces three genotypes. Using the binomial expansion that NCERT applies to the monohybrid cross, (½R + ½r)² expands to ¼ RR + ½ Rr + ¼ rr. The genotypic ratio is therefore 1 RR : 2 Rr : 1 rr.

Up to this point, snapdragon behaves identically to Mendel's tall–dwarf pea cross. The divergence appears only when genotype is translated into phenotype. In the pea cross, the ¼ TT and the ½ Tt plants are all tall and cannot be told apart externally, so they merge into a single phenotypic class of ¾ tall, against ¼ dwarf — the 3:1 ratio. In snapdragon, the ¼ RR are red, the ½ Rr are pink, and the ¼ rr are white. No two genotypes share a phenotype, so the three classes stay separate: 1 red : 2 pink : 1 white.

This is the conceptual heart of the subtopic, and the line NEET examiners return to repeatedly. The 3:1 ratio of a Mendelian monohybrid cross is fundamentally a phenotypic compression of an underlying 1:2:1 genotypic ratio. Complete dominance hides the heterozygote inside the dominant class. Remove that masking — give the heterozygote its own appearance — and the hidden 1:2:1 simply becomes visible. Incomplete dominance does not create a new ratio; it reveals the genotypic ratio that was always there.

Why R fails to mask r — the dosage explanation

NCERT pairs incomplete dominance with an explanation of what dominance actually means at the level of the gene. Every gene carries information to make a product, often an enzyme. A diploid plant carries two copies of each gene. The functional R allele codes for a working enzyme that catalyses the production of red pigment in the petals. The r allele is a modified form that produces either a non-functional enzyme or no enzyme at all, and so makes no pigment.

A homozygous RR plant carries two functional copies and synthesises a full dose of pigment, giving deep red flowers. A homozygous rr plant carries no functional copy, makes no pigment, and the flowers stay white. The heterozygote Rr carries exactly one functional copy. In many traits a single functional allele still makes enough product to give the full dominant phenotype — that is ordinary complete dominance. But in snapdragon flower colour, one functional copy produces only about half the pigment of two copies, and that half-dose is visibly less intense. The petal therefore looks pink rather than red.

This dosage view delivers a useful insight: dominance is not an absolute property of an allele. It depends on whether one functional copy is enough to build the full phenotype. NCERT makes exactly this point with starch synthesis in pea seeds. The gene has alleles B and b; BB seeds make large starch grains and round seeds, bb make small grains and wrinkled seeds. For seed shape, B behaves as a fully dominant allele, since Bb seeds are round. But the starch grains in Bb seeds are of intermediate size, so for the phenotype "starch grain size", the very same B and b alleles show incomplete dominance. The same allele pair can be completely dominant for one trait and incompletely dominant for another, depending on which phenotype you choose to measure.

Incomplete dominance is not blending inheritance

A pink F1 between red and white parents looks superficially like the old, discredited "blending" theory of heredity, in which parental traits supposedly mix permanently like paints. Incomplete dominance decisively refutes blending. If colour truly blended and fused, a self-cross of two pink plants could never recover pure red or pure white — yet the snapdragon F2 produces both parental colours again, cleanly. The alleles R and r remain discrete, particulate units; they segregate intact into gametes and reassemble. Only the visible expression of the heterozygote is intermediate. The hereditary factors themselves never blend.

Two further points sharpen the comparison for the exam. First, incomplete dominance is an exception to the Law of Dominance only. The Law of Segregation holds without exception — NCERT and NIOS both treat it as universal — which is why the F2 genotypes remain 1:2:1. Second, the test cross becomes redundant in incomplete dominance. Mendel needed a test cross because he could not tell a TT plant from a Tt plant by sight. In snapdragon, RR is red and Rr is pink: genotype is read straight off the phenotype. A pink plant is unambiguously heterozygous.

Briefly contrasting with codominance

Codominance is the other classic deviation from the Law of Dominance, and NEET frequently asks the two together, so the distinction must be airtight. In incomplete dominance the heterozygote shows a single blended, intermediate phenotype — pink, which is one new colour, not red-and-white side by side. In codominance the heterozygote expresses both parental phenotypes fully and separately, with no blending and no intermediate. The NCERT example of codominance is the AB blood group: a person with genotype I^AI^B has red blood cells carrying both A-type and B-type sugars at full strength, because both alleles express completely and simultaneously. There is no "intermediate sugar". The short rule: incomplete dominance blends into one new phenotype, codominance shows both phenotypes at once.

Worked examples

Common confusion & NEET traps

Most marks lost on this subtopic come from three predictable confusions: mixing up incomplete dominance with codominance, wrongly claiming the Law of Segregation is violated, and forgetting that the 1:2:1 here is a phenotypic ratio, not merely a genotypic one.