a) Incomplete or partial dominance:

• When some gene fail to show complete dominancy and both alleles show their expression and the hybrid appears intermediate between the two parents. This is called incomplete dominance.
• It was first observed by Correns in 4 O’clock plant Mirabilis jalpa.

• In F1 generation , all plants are pink in color. This is intermediate between red and white color.

b) Co-dominance:

• In Co-dominance , both the alleles of a gene equally express in F1 generation.
• Such alleles are called dominant and the phenomenon is called co-dominance.
• Eg: AB blood group in a man is a good example of co-dominance. When homozygous blood group I^AI^A marry with homozygous blood group IBIB then produced offspring have blood group AB (IAIA).
• Here, both IA and IB alleles are fully expressed.

• In F1 generation, all blood group is AB.

• Genotype: 1:2:1
• Phenotype: 1:2:1(or 3:1).

c) Overdominance:

• It is the is the condition wherein a heterozygote produces a phenotype more extreme or better adapted than that of the homozygote.
• Eg: Sickle cell anemia in human, eye color in Drosophila