The factor that is responsible for the phenomenon of the blood group B being almost absent in Native Americans whose ancestors arrived in very small numbers about 10,000 years ago would be called the 'founder effect'.
The difference between the meanings of combining forms spondyl/o and vertebr/o is spondyl/o and vertebr/o both mean vertebra. There is no difference because they are both vertebra.
Spondyl/o is a combining form meaning vertebrae. The two combining forms for vertebrae bones of the spine, are vertebr/o and spondyl/o.
