What makes a Giant Leopard Gecko?

The “Giant” mutation was uncovered by Ron Tremper in 1999, when he observed an unusually large hatchling in his albino project.  Two years later, the famous “Moose” was born, reaching a weight of 156 grams at three years old.  Some of his offspring have reached the 170 gram mark.  Within four years it was determined that this was indeed a genetic, and predictable, trait.  Being incomplete dominant, there are two possible Giant forms.  The Giant, which is considered heterozygous for the trait, and Super Giant, which is the homozygous, or pure form.  A true Super Giant is nearly unmistakable compared to other animals at the same age, especially beyond the first year.

While weights are generally accepted as a barometric indicator of this gene, they are NOT reliable, and in my opinion mean absolutely NOTHING.  The mutation results in increased length, creating an elongated appearance, especially in the torso and head.  Tails can also get longer than normal.  Normal, non-giant leopard geckos are capable of reaching the magical weights you may see thrown around on the Internet and in books, but clearly they are not the same.  Moral of the story, don’t assume your animal is a Giant because of its weight, and at the same time, don’t assume your 80 gram male could not be a Super Giant.  Take into consideration the age, length, and above all else, genetic history.  And here’s a hint… look very closely at your animals, and compare them side by side with non-giants as much as possible.  “Giants” do look different, albeit sometimes in a very subtle way.  I would entertain the notion that this gene is actually recessive, and not incomplete dominant.

As such, the “Giant” trait can be a bit ambiguous, and confusing.  If you’re unsure of the genetics of your animal, the best thing to do is test breed it, just as you would with any recessive trait.  The following chart shows the predicted outcomes, simplified out of punnet square form.

Super Giant X Super Giant = 100% Super Giant
Super Giant X Giant = 50% Giant, 50% Super Giant
Super Giant X Non-Giant = 100% Giant
Giant X Giant = 25% Non-Giant, 50% Giant, 25% Giant
Giant X Non-Giant = 50% Non-Giant, 50% Giant