If I remember correctly, the parents only produced one Jag. It was only when the founder was bred that more were produced. If Jan had produced more from the normal x normal pairing, he wouldn't be pondering the possibilities of the gene being co dominant. If it is a co dominant trait, it would have been spontaneous mutation that created the first Jaguar's appearance.
:Hmmm....so what about the parents continuing to produce Jags after that first breeding? Spontaneous mutations are like one in a billion chance. And you think it happened more than once with the same animals in the same generation?