I don't know exactly what you mean but the size of a group affects its defense because there is a larger group. Therefore, more people would be protecting it, adding more to its defense.
More things in a group would equal a bigger and better defense. Meaning that you would have more protection and be able to cover a larger area overall as well.
If you have one sickle cell gene, you have sickle cell trait and you will not develop the sickle cell disease. If you inherit two sickle cell genes, you have sickle cell anemia.
