incomplete dominance
after Mendel, several cases were recorded where f 1 hybrids were not
related to either of the parents but exhibited a mixing of characters
of two parents. It means that two genes of the allelomorphic pair
are not related as dominant and recessive, but the dominant gene in
heterozygous condition has reduced expression, so that each of them
expresses itself partially. This is called incomplete dominance.
Example: In Mirabilis jalapa (four-o’-clock plant) and Antirrhinum
majus (snapdragon or dog flower), there are two types of flower
colour in pure state: red and white.
When the two types of plants are crossed, the hybrids or plants of
f 1 generation have pink flowers. If the latter are selfed, the plants
of f 2 generation are of three types–red, pink and white flowered
in the ratio of 1 : 2 : 1. The pink colour apparently appears either
due to mixing of red and white colours (incomplete dominance) or
expression of a single gene for pigmented flower which produces only
pink colour (quantitative inheritance).
Codominance
If two alleles of a single gene are responsible for producing two distinct and detectable gene products, a situation different from
incomplete dominance or dominance-recessiveness arises. In such a case, the joint expression of both alleles in a heterozygote is
called codominance.
Example: Codominance can be illustrated by MN blood group in humans. MN antigens do not elicit strong immune reactions,
hence often not considered in blood transfusion.
The MN blood group system is under the control of an autosomal locus found on chromosome 4, with two alleles designated as LM
and LN. because humans are diploid, three combinations are possible, each resulting in a distinct blood type :
Genotype Phenotype
LMLM M
LMLN MN
LNLN N
as predicted, a mating between two heterozygous MN parents may produce children of all three blood types in the ratio of 1 : 2 : 1.
This example shows that codominant inheritance is characterised by distinct expression of the gene products of both alleles.Difference between incomplete dominance and codominance
Though the f 2 phenotypic ratios are identical in both incomplete dominance and codominance, there is difference in the
correlation of alleles. In case of incomplete dominance, the two alleles of a pair interact producing an intermediate phenotype,
whereas in codominance both the alleles of allelic pair contribute equally to the phenotype and the expressed phenotype is
a combination of the phenotypic traits produced by either of the alleles in homozygous condition.Multiple alleles
for any gene, the number of alleles within members of a population of individuals is not necessarily restricted to two. When three or
more alleles of the same gene are found, multiple alleles are said to be present, creating a characteristic mode of inheritance. The
multiple alleles can be studied only in populations. a diploid organism has, at most, two homologous gene loci that may be occupied
by different alleles of the same gene. However, among members of a species, many alternative forms of the same gene can exist.
Example : ABO blood type alleles in humans form an example of multiple alleles. The abO locus has three common alleles : IA, IB and i.
IA and IB are codominant (IAIB heterozygotes have both a and b antigens on their red blood cells) and i^ is recessive (ii homozygotes
have no antigens on their red blood cells; IAi and IBi^ heterozygotes have a and b antigens, respectively, on their red blood cells).