Page 8 - Lesson Notes- Principle of inheritance & Variation

Co-dominance

It is the phenomenon of two alleles lacking dominance-recessive relationship and
both expressing themselves in the organism.

Human beings, ABO blood grouping are controlled by gene I. The gene has three

A, B
A, B
alleles I I and i. Any person contains any two of three allele I I are dominant
over i.
The plasma membrane of the red blood cells has sugar polymers that protrude from its

surface and the kind of sugar is controlled by the gene.

A
B
When I and I are present together, both express their own types of sugars because of
co-dominance.

Incomplete Dominance Co-Dominance

Effect of one of the two alleles is more Effect of both the alleles are equally
1. 1.
conspicuous. conspicuous.

It produces a mixture of the expression of There is no mixing of the effect of the
2. 2.
two alleles. two alleles.

The F1 does not resemble either of the
3. 3. The F1 resembles both the parents.
parents.

E.g.: Flower colour in dog flower. E.g.: ABO blood grouping in humans,

Multiple Alleles

They are multiple forms of a medelian factor or gene which occur on the same gene locus

distributed in different organisms in the gene pool with an organism carrying only two
alleles and a gamete only one allele. ABO blood grouping also provides a good example of

multiple alleles.

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