What is incomplete dominance and codominance : Examples & Exceptions


After “Mendalism” now read “Exceptions of Mendelism : incomplete dominance and codominance” with example and conclusions.

Exceptions to Mendelism:

 Exception of dominance: there are two exception of law of dominance.

(A) Incomplete dominance
(B) Co dominance

Incomplete dominance

Both factors such as dominant and recessive are present in incomplete dominance but dominant factor are unable to express their character completely resulting different type of generation is formed which is different from the both parents.

Incomplete dominance was first discovered by Correns in Mirabilis jalapa this plant is called as ‘4 O clock plant.

(a) Three different types of plant are found in mirabilis on the basis of flower colour such as red, white and pink. When plant with red flower is crossed with white flower plant than pink flower obtained in f1 generation, the reason of this is that the genes of red colour incompletely dominant over the genes of white colour.

When f1 generation of pink flower is self pollinated then phenotypic ratio of F generation is red, pink, White =1:2:1 in place of normal monohybrid cross = 3:1

The ratio of phenotype and genotype of f2 – generation in incomplete dominance is always same.

(b) Antirrhinum majus:- incomplete dominance is also seen in this plant is also known as Snapdragon or dog flower.

(c) Andalusian fowls:- Incomplete dominance is present for their feather colour when a black colour fowl is crossed with a white colour fowl the colour of f1 generation is blue.


In this phenomenon both the gene expressed equally for a particular character in f1 hybrid progeny. There is no blending of characters whereas both the characters expressed equally. Ratio of phenotype as well as genotype is same & 1:2:1 in co dominance.

Note:– In incomplete dominance characters are blended phenotypically while in co dominance both the genes of a pair exhibit both the characters side by side and effect of both the character is  independent from each other

Examples of co dominance:

(ii) AB blood group in heritance (IAIB)
(iii) Carrier of sickle cell anaemia (HbA Hbs)

Conclusion of segregation

There is no exception of law of segregation.  The segregation is essential during the meiotic division in all sexually reproducing organisms. (No disjunction may be exception of this law)

Exception on conclusion of independent assortment

The law of independent assortment is most criticized; linkage is the exception of this Linkage – collective inheritance of character is called linkage .Linkage first time seen by Bateson and Punnett in Lathyrus  odoratus and gave coupling and repulsion phenomenon but they did not explain the phenomenon of linkage .

Sex linkage was first discovered by Morgan in drosophila & coined the term linkage. He proposed the theory of linkage.

Linkage and independent assortment can be represented in dihybrid plant.


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