Blood types are determined by a special type of inheritance called codominance . Everybody inherits two different alleles , or forms of a gene, from each parent to make up your genotype , or genetic makeup. Alleles can either be dominant or recessive. If you inherit a dominant allele for a trait, that dominant allele will control how you display that trait. In order to display the recessive trait, both alleles in your genotype have to be the recessive allele.
A, B, and O are all different types of alleles for blood types. A and B are codominant, meaning that both are dominant forms of an allele. Therefore, if you inherit both an A allele and B allele, you will have an AB blood type. O is recessive. If you inherit two O alleles, then you will display an O blood type. If you inherit an A and an O, then your blood type would be A, and if you inherit a B and an O, then your blood type would be B.
Blood types also have another component to them, called the rhesus factor , also known as the Rh factor. The Rh factor is a protein that can be found on red blood cells. If you have this protein, then your Rh factor would be positive. If you don't, it would be negative (an A+ blood type would have the protein and an A- would not). A positive Rh factor is dominant, therefore if you inherit a positive from either parent, you would have a positive Rh factor. You can only have a negative Rh factor if you inherit negative alleles from both of your parents.