OK, I should probably shut up, but I have to say that genetics are complicated. It's possible for a line to carry a recessive gene (say for a protein chain deficiency) and it will not be expressed for many generations. That's because each dog will still have one good gene that overrides a defect.
The line will continue fine until two dogs who both carry it (say half-siblings) breed. Then each of their pups will have a 1:4 chance of not carrying the gene at all, 2:4 chance of carrying the gene and being totally healthy (but now they are both carriers themselves) and a 1:4 chance of getting both copies of the gene and then expressing symptoms.
Plus then there is always the possibility of a spontaneous gene mutation that NO ONE could have seen coming, no matter what the lineage.
I'm NOT saying this is what happened in THIS case. Just that because only one pup out of a litter is sick and the rest are healthy and because this type of thing hadn't happened in the line before now doesn't rule out that there is a genetic component. |