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To keep any hybrid vigour going, you need to keep mating the original two breeds.

But while the UK Kennel Club has been registering crossbreeds on its database for more than 50 years, record-keeping of their parentage has been lacking, making it difficult for scientists to assess their hybrid vigour.

Hybrid vigour is the “mirror image” of inbreeding depression, Nicholas writes, weakening of certain features as a result of excessive breeding with close relatives.

As a general rule, the offspring of two different species or breeds has greater genetic diversity, creating hybrid vigour – enhanced fertility, health and survival.

But due to tainted data overall, Nicholas stresses the importance of keeping careful records so canine hybrid vigour can be scrutinised.