Short notes for NEET & CBSE on Incomplete Dominance and Co-dominance

This is a follow up to the previous article which was an introduction to genetics and the laws of inheritance. Read on to find crisp notes on Incomplete Dominance and Co-dominance, which will surely be helpful in NEET as well as CBSE!

Gregor Mendel  happened to use pea plants, which happened to have a number of easily observable traits that were determined by just two alleles.  For all the traits, one allele happened to be dominant for the trait & the other was a recessive form.  Things aren’t always so clear-cut & “simple” in the world of genetics, but luckily for Mendel (& the science world) he happened to work with an organism whose genetic make-up was fairly clear-cut & simple.

Incomplete Dominance:

With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.


RED Flower x WHITE Flower —> PINK Flower

Eg: Mirabilis jalapa (Four o’clock plant),

       Antirrhinum majus (snapdragon or dog flower)

Here, when a cross is made between red flowered (RR) and white flowered (rr) varieties, F1 (Rr) progeny produced is an all pink flowered. When these F1 pink flowers are self pollinated or crossed among themselves to raise F2 generation, they produce red (RR), pink (Rr) and white (rr) flowers giving 1:2:1 ratio. This phenotypic ratio is identical with genotypic ratio because heterozygotes are phenotypically intermediate between two homozygous types.


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Gene Interaction:

Gene interaction is the influence of alleles and non-alleles on the normal phenotypic expression of genes.

1. Intragenic (inter-allelic)- In this case two alleles (located on the same gene locus on two homologous chromosomes) of gene interact in such a fashion to produce phenotypic expression other from normal dominant recessive phenotype e.g., co-dominance and multiple alleles.

2. Intergenic (non-allelic)- Here two or more independent genes present on same or different chromosomes interact to produce a new expression e.g., epistasis, complementary genes, supplementary genes, duplicate genes, inhibitory genes, lethal genes etc.


Sometimes, neither of a pair of alleles is completely dominant or completely recessive. When the dominant character is not able to suppress, even incompletely suppress the recessive character and both the characters appear side by side in F1 hybrids, the phenomenon is called co-dominance. In this case the F1 generation resembles both parents.

red x white —> red & white spotted

Eg: ABO blood typing in humans.

In humans, there are 4 blood types (phenotypes): A, B, AB, and O

Blood type is controlled by 3 alleles: IA, IB, IO (the base letter = I stands for immunoglobulin)

IO is recessive, two IO alleles must be present for the person to have type O blood

IA and IB are codominant but both are dominant to IO.


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