The chickens come home to roost
The one-to-another joining of hemoglobin tetramers leads to the formation of giant filaments. These in turn deform and rigidify the red blood cell. Lacking their usual flexibility, the red blood cells are torn while navigating tiny vessels, generating large amounts of debris and depleting the body's oxygen delivery systems (hence anemia). Strikingly, it's not all bad--heterozygotes (having one good and one mutant beta globin gene) have increased resistance to malaria. But that's a story for another day.
Use these buttons for further investigation of the structure if you wishTrace