to clarify it what is her theory, this is taken from it, it is only a part of it, please read the whole article:
I have begun to suspect that that nice long distance between the tail base and the pattern break might be an indicator of outbreeding, or of good genetic health and general robustness. I think that because some "sickly" hatchlings seem to have a pattern break much closer up the tail to the body, and fail to thrive. Some adults who have been beset by illnesses also have that pattern, suggesting they are less robust than others and need more attention to keep healthy
Kathryn W. Tosney, Professor of Biology, The University of Michigan
I talk about up the tail as being toward the tip, that is NOT what Kathryn meant when she states closer up the tail as she further states close to the body.
Her theory and if you read the whole article by her that is posted below:
Tail pattern breaks between the tail base and the 5th-6th ring = inbreeding
Lower toward the tip of the tail = may indicate outcrossing
She further points out that this DOES NOT indicates illness or problems in a dragon, just the possible degree of like genetics markers in a given individual dragon.. hence you could have siblings showing perfect tail patterns, or one clutch showing none, another clutch from the same parents showing many or some.
It was stated as a possible guide, all things being equal in a dragon, she would select the one without tail pattern breaks.... even lower towards the tip of the tail that she thinks may indicate outcrossing, would still have past genetic lines of inbreeding.
This pictures shows babies with many tail pattern breaks all along the tails, and they are thought to have been from related parents, an accident of breeding at to early an age before they were separated.
All I can say for sure is dragons that we have examined that have failure to thrive problems almost always have multiple breaks in the tails... that does not mean that all breaks would equate to an ill dragon, just that they may have like genetic markers. But combined with a know illness or defect, it can be a marker as almost all that have problems have tail pattern breaks.
Does that clarify any?

Dr Kathryn Tosney's Tail Break Pattern Theory