1. Incomplete Dominance
Carl Correns when working on Mirabilis jalapa the four o’ clock plant observed the more common wild type as having bright red flowers (RR) while the less observed recessive variety had white flowers (rr). When two such from pure lines were crossed, the offsprings of F1 did not show the same dominant phenotype as the F1 offspring of Mendel’s Monohybrid experiment.
The heterozygote Rr at F1 was neither ‘red’ nor ‘white’ in colour but instead showed andin between’ expression and were pink in colour. This was a deviation from the expected dominant phenotype at F1 as described in Mendel’s experiment. Again when these Pink – flowered varieties of F1 were selfed, the progeny at F2 did not show the expected 3 dominant: 1 recessive ratio but instead showed 1 : 2 : 1 (Red : pink : white), with the new intermediate colour appearing in larger numbers, than either parental phenotype.
The presence of red colour is attributed to production of pigment (with RR alleles). The rr alleles therefore lack colour due to lack of pigment. The heterozygote Rr therefore possibly result in production of less quanta of pigment and less colour. An incomplete dominance allows the allele r to express itself, resulting in an intermediate phenotype.
2. Co-Dominance
Each allele in the gene pair expresses itself and this is seen as a new, definite phenotype trait due to the interaction of both alleles.
Example: Coat colour in cattle is said to be controlled by the alleles R and r. ‘R’ is the dominant allele, which results in a dark coat colour. The recessive ‘r’ results in a pale or white coat colour in the cattle.
In the heterozygous form RR – a Red-Brown colour is observed in the animal (parental type).
The recessive alleles rr, in the homozygous form results in a White colour. A cross of parental types RR dominant homozygotes with rr recessive homozygotes produces heterozygotes ‘Rr’, which have patch of white and brown, since both alleles manifest their phenotype. Unlike the phenomenon of incomplete dominance, an intermediate colour is not observed but some cells show the dominant dark trait while others show the recessive white trait. (Red-Brown) RR × rr (white)Rr (patched).
A second example of co – dominance is lentil seed cover. There is a spotted variety of lentil (cs cs ) which has white spots on a full green background and a second variety of green spots on white background– Dotted (cd c d ). In a cross between two such parental types : Pcs c s × cd c d F1 cs c d
The F1 heterozygote showsboth spotted as well as dotted – seeds having both green and white dots – Since both cs and cd alleles express themselves.
Example of Co-dominance
In humans, complex membrane anchored molecules that distinguish different types of red blood cells, are shown to exhibit the co–dominant inter-relationship between the alleles. These RBC surface antigens actually determine blood group phenotype in man.
I o has been designated as the allele that encodes for the basic ‘H’ protein that makes up the major portion of this RBC – Antigen.
I a and Ib are variant forms of the basic allele which encode for the enzyme β – galactosidetransferase. The enzyme recognizes a specific glycoside moiety as a substrate and transfers it to the terminal end of the glycoprotein constituting the antigen. Each allele I a and Ib therefore encode for variant enzymes that differ in their substrate recognition and binding activity and therefore adds a different terminal sugar to the RBC antigen. Thus both the antigen differ only minutely. It is this minute difference that is identified by the antibody (immunoglobulins) as “foreign”.
Here, there are variant alleles encoding indirectly for different antigens:
Antigen A – termed as blood group A (allele IA )
Antigen B – blood group B (allele IB )
Both Antigen A and B – blood group AB (allele IA I B )
Absence of Antigen : Antigen ‘O’ – blood group O (allele IO ) (terminal glycoside moiety absent) There exists a complete dominance between the alleles IA and IO and IB andIO such that IA is dominant over IB is dominant over IO
Blood group O occurs therefore in the absence of alleles IA , IB when the genotype has alleles IO I O .
However, there is a co – dominant relationship between alleles IA and IB such that both alleles when present together, do not mask each other but both alleles express their products resulting in the presence of both A andB antigenic types, thus forming a new phenotype blood group AB. It is clear therefore, that the co – dominant interaction results in a novel phenotype, where both allelic expressions can be observed.