My vet is the one who suggested that I breed mine. She says shes never seen such a calm and friendly demeanor in a Jack Russel and says the breed could use more of it.
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If I could breed the current dog, it would be wonderful. Intelligent, healthy, with a great personality, and good disposition. Sometimes I think we fix some of them too soon.
As with human siblings of the same parents, there are variations among the siblings in regards to intelligence, personality and disposition. These traits are not necessarily hereditary.
English Shepherds to be used for breeding should be OFA or PennHIP tested. The dog being considered for breeding purposes may have perfect hips, yet carry the gene for hip dysplasia, which is genetically inherited.
Wikipedia tells us that 15% of all English Shepherds carry the MDR1 gene mutation, a potentially life-threatening condition. Potential breeding stock should be tested for the MDR1 gene mutation. Again, the dog selected for breeding may not have this condition, but may be a genetic carrier and pass it on to the puppies.
In addition to all the absolutely excellent advice given by rintense, I'd add studying and understanding genetics to the list of important considerations.