Codominance and incomplete dominace | class 10
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This lecture is about
Co-Dominance And Incomplete Dominance
After the discovery of Mendel’s work, scientists began experiments on the genetics of various
organisms. These experiments proved that all the traits in organisms do not follow Mendel’s laws.
For example, it was found that there are many traits which are controlled by more than one pair of
genes. Similarly for many traits, there are more than two alleles in a gene pair. Co-dominance and
incomplete dominance are two examples of such deviations from Mendel’s lawsCo-dominance is the situation where two different alleles of a gene pair express themselves
completely, instead of showing a dominant-recessive relationship. As a result, the heterozygous
organism shows a phenotype that is different from both homozygous parents.
An example of co-dominance is the expression of human blood group AB. The ABO blood group
system is controlled by the gene ‘I’. This gene has three alleles i.e. IA, IB and i. The allele IA produces
antigen A in blood and the phenotype is blood group A. The allele IB produces antigen B in blood
and the phenotype is blood group B. The allele i does not produce any antigen and the phenotype
is blood group O. The alleles IA and IB are dominant over i. When there is a heterozygous genotype
of IA IB, each of the two alleles produces the respective antigen and neither of them dominates over
the other
In-complete dominance is the situation where, in heterozygous genotypes, both the alleles express
as a blend (mixture) and neither allele is dominant over the other. As a result of this blending, an
intermediate phenotype is expressed. Following is the familiar example of incomplete dominance In Four ‘O’ clock plant, the trait of flower colour is controlled by two alleles (let us say them R and r).
The true breeding plants RR and rr have red and white flowers, respectively. When a homozygous
red flowered plant (RR) is crossed with homozygous white flowered plant (rr), the heterozygous (Rr)
plants of F1 generation produce pink flowers (pink is a blend of red and white colours). This result
clearly indicates that neither of the red flower allele (R) and white flower allele (r) is dominant.
However, when two heterozygous plants with pink flowers (Rr) are crossed, F2 generation shows
phenotypes of red, pink and white flowers in the ratio 1:2:1.