Two of these chains are identical alpha units, and the other two chains are identical beta units. And what this means is in any given hemoglobin molecule, we'll always find a ratio of one alpha unit to one beta unit. So one alpha polypeptide always combines with one beta polypeptide unit to form that hemoglobin molecule, in which we have two dimers, two alpha beta dimers. Now, interestingly enough, if we examine and studied the human genome molecule, so all the DNA molecules found inside the human cell, we're going to find four genes that code for the alpha unit, but we're only going to find two genes that code for the beta unit. So for some reason, inside our genome, we have twice as many genes that code for the alpha subunit than the beta subunit. Now, why can that be a problem?