Deer are mammals of the family Cervidae. They include species with bony antlers that are shed and regrown, and their browsing can strongly influence vegetation. Family membership, external appearance and the ancestry of an individual are related but different kinds of information.
A genetic study of red and introduced sika deer in Scotland combined microsatellite markers with mitochondrial DNA. Two broadly distinct genetic groups persisted, yet introgressed alleles were detected where their distributions overlapped. [ref]
Introgression through rare hybridization: A genetic study of a hybrid zone between red and sika deer (genus Cervus) in Argyll, Scotland by Simon J. Goodman and others. 1999, abstract: eleven microsatellite markers, mitochondrial DNA and introgression
Thus recognizable groups can exchange some genes without becoming immediately indistinguishable. Appearance alone need not reveal every episode of ancestry across a hybrid zone. New to topics? Read the docs here!