Transitional vertebrae are a congenital malformation of the spine that occur at the junctions of major divisions of the spine (usually between the thoracic and lumbar vertebral junction and the lumbar and sacral vertebral junction). Transitional vertebrae take on anatomic characteristics of both divisions of the spine it occurs between. The most common type of transitional vertebrae in dogs is in the lumbo-sacral area where the last lumbar vertebral body takes on anatomic characteristics of the sacrum. Transitional vertebrae are usually not associated with clinical signs and the dog can be used in a breeding program. The OFA recommends breeding the dog to another dog that does not have transitional vertebrae.
I had never heard of this when I prelim'd Mr Tibbs 7 years ago, and questioned OFA on it as it was shown on his prelim certificate, he was OFA excellent and elbows were normal. After a discussion with OFA, I carted those x-rays everywhere and showed them to ortho vets at every opportunity including a few who had actually been on the OFA panel at one time or another and not a single one of them could identify where or why the OFA panel at that time saw the "transitional vertabra"
At 8 years old Mr Tibbs has never taken a lame step and is still as rough and tumble and sound as ever and not a single one of his children has ever had a problem or had this phenomenon show up on an OFA report. It remains a mystery to me but not one I have spent much time worrying about. I do not consider it a reason to neuter and place an otherwise sound dog.
