1. Co-dominance occurs when both alleles present on the gene express themselves equally in the progeny. In this, F1 has the phenotype that resembles both the parents. For example, in human blood group alleles, IA and IB are codominant, so when they come together, the individual has AB blood group.
2. Incomplete dominance occurs when a dominant allele does not completely mask the expression of recessive allele and the phenotype of progeny shows a blended expression of both the alleles. The heterozygote formed in this case has a different phenotype from homozygous for either of the alleles.
It was first studied in the Mirabilis jalapa. The phenotype of the F1 generation was intermediate of both the parents.
Cross: red flower (RR) X white flower (rr)
F1 generation: Rr (pink)
rr
F2 Genotypic ratio= 1:2:1 (RR: Rr: rr)
F2 Phenotypic ratio= 1:2:1 (Red : Pink : White).
3. Complete dominance occurs when the dominant allele completely masks the expression of the recessive allele.
4. Pseudo-dominance occurs when the heterozygous recessive phenotype expresses itself by deleting the dominant gene present on the chromosome.