Blood type is an example of codominance
incomplete dominance
complete dominance
sex-linked traits

Codominance means that neither allele can mask the expression of the other allele. An example in humans would be the ABO blood group, where alleles A and alleles B are both expressed. So if an individual inherits allele A from their mother and allele B from their father, they have blood type AB

Closely related to incomplete dominance is codominance, in which both alleles are simultaneously expressed in the heterozygote. We can see an example of codominance in the MN blood groups of humans (less famous than the ABO blood groups, but still important!)

Genes in Families

The human ABO blood group system exhibits codominance. The system consists of three alleles A, B, and O. Both A and B are dominant in relation to O, and therefore blood group A can have the genotype AA or AO. Blood group B can have the genotype BB or BO.

(≧▽≦)

the correct answer is 2 and 4