I hadn't heard this phrase before so I had to look it up. In short, it seems to be a sort of wive's tale based on an understanding that some collies are sensitive to certain medications, and in turn associated with those collies having white feet. I don't know if there's any supporting evidence that the gene that produces white feet in dogs is related to the mutation of the MDR1 gene that causes the drug sensitivity. And with other breeds being identified as having the mutation it seems more unlikely.
What is the MDR1 gene?
The MDR1 (Multi-Drug Resistance 1) gene exists to facilitate in proteins that move toxins out of the brain. The cases we're worried about here is when a mutation in the MDR1 gene occurs that inhibits those proteins from moving certain toxins out of the brain.
Now I'm not well-versed in genetics, but my understanding is that all dogs have the MDR1 gene and the mutation is passed on through breeding in the same way other traits are. With each parent passing on a copy of the gene to their offspring, and whether one or both parents pass on a mutated copy decides whether the child also has the mutation.
MDR1 mutation and white feet?
It seems as though herding breeds are more susceptible to this mutation than other breeds because it was unintentionally selected for during their breeding history. The gene was only just identified as the cause of the reactions to medication in 2001, so they were being bred long before the gene was identified as the root cause, and well before gene testing was a thing.
In my opinion, the white feet correlation is just a false correlation built from anecdotes of which dog's have shown symptoms of the mutated gene. So many herding breeds have mottled coats that it's easy to put together anecdotes of the ones with white feet showing signs of drug sensitivity. I've not found anything that defines a link between genes that define coat color and/or pattern and the MDR1 mutated gene. And with other breeds of dogs being identified as having the mutated gene, it seems unlikely that it's a good indicator of your dog having the mutation.
Should you get your dog tested?
It's recommended that you get your dog tested for this gene mutation if they're a herding breed, or mixed with a herding breed. Since herding breeds are known to have a higher chance of passing on the mutation.
I would personally suggest getting mixed/unknown breeds tested just in case, since they could have history with the mutation. With purebred non-herding breeds it seems like you can get away with not testing as the risk of the mutation is comparatively low.
I assume all veterinarians can facilitate in genetics testing, but there are some other resources available as well. You can order test kits from websites like AnimalGenetics.us where they will test and send you the results directly, or universities like Washington State University.
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