This is because you get one set of genes from each parent. In order for a pair to potentially produce Platinum JD's, both the mother and the father have to have at least one copy of both the blue and gold genes. In this case, the mother has a double gold gene, but no blue gene and the father has a single blue but no gold. In school they show you how to figure this out using a punnet square. This is more complicated as you are looking at two independent recessive genes - the same logic applies, but more complicated. You could do the same thing with 3, 4, or 5 recessive genes, just makes your head hurt!
More info on this in this other thread:
http://www.monsterfishkeepers.com/f...Platinum-Electric-Blue-Dempsey-breeding-chart
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