Answer:
A dominant gene will hide the recessive gene. It is shown as a capital letter in the gene. (i.e. Rr, R is the dominant gene) In blood type, A is dominant over O. So if a person has Ao for their genes, they will have A type blood.
A recessive gene will be hidden unless there are two of the same recessive gene. It is shown as a lowercase letter in the gene (i.e. Rr, r is the recessive gene). In blood type, O is the recessive allele, so you cannot have O type blood unless you have 2 O alleles: oo.