Can you breed two different size dogs together?

When you mix two dogs with very different personality types and different hardwired behaviours, it’s difficult to know what you are going to get – and it is impossible to know that in an eight-week-old puppy. Just like with coat type, you have no idea how the combined size and shapes of the two breeds will mix in any one puppy. If you look at a litter of ‘designer crossbreeds’ – or even more dramatically, look at them as adults – you are unlikely to see many similarities. Some Cockapoos look like Spaniels, others like Poodles, and others somewhere in the middle. Size, shape, coat, colour… all are variable. This means you can’t predict what your puppy is going to grow into and it can be hard to predict future grooming schedules and behaviour. Just to confuse matters even more, often a crossbreed will turn out larger than either of the parents. In addition, some of these hardwired behaviours don’t mix well – for example, any of the Gundog breeds who were designed as retrievers crossed with any of the breeds who are a little more competitive, can easily produce a dog more likely to resource guard their valuable prize than give it up.

Dogs inherit genes at random. In dogs, parents pass down 50% of their genetic makeup to their offspring. The puppies, however, inherit segments of the available DNA at random. The result of this process, known as genetic recombination or genetic reshuffling, is that the actual genetic composition of puppies in the litter varies. But in mixed breed dogs—which are genetically diverse—the variation in genetic composition typically yields different breed percentages, physical traits and predispositions. A litter of puppies can have different fathers. Female dogs produce multiple eggs at one time—that’s why they typically give birth to litters rather than single puppies. As a result, puppies from the same litter may actually have different fathers. On average, siblings who share the same parents are approximately 50% genetically related. But if more than one male fathers a litter, the half-siblings puppies are only ~25% genetically similar. This can produce very different breed ancestry results—and dramatically different looking puppies—within a single litter.