Here is some info on metabolic bone disease that I wrote a while back. I think Heartmountain is exactly right and you should get him/her to the vet ASAP. Also remove any climbing furniture for exactly the reasons he stated. Especially when he starts feeling better, he may want to climb before he is ready.
Here's the old post:
MBD (Metabolic bone disease) is a catch-all phrase for a myriad of syndromes.
All of these syndromes are characterized by a decrease in bioavailable calcium. This can be because of several reasons: they are not fed a diet of calcium rich foods and/or are not supplemented with calcium, they are not exposed to UVB and therefore cannot synthesize vitamin D3, the animals have a negative calcium: phosphorous ratio (phosphorous binds calcium to form calcium phosphate which is not a bioavailable form of calcium), and finally the less common causes of the decreased amount of calcium is a disruption of the animal's ability to synthesize vitamin D3 (even if supplemented or exposed to UVB) due to kidney, liver,intestinal, thyroid, or parathyroid disease. (ref. Mader, et al 1996)
With that said as to your question, no beardies are not more or less susceptible to MBD. Mostly because it is primarily a dietary/husbandry related disease. It can be a heritable predisposition for offspring of individuals with organ dysfunction that is congenital. Knowing your animals and your care ( I am sure you have ruled out an bad UVB bulb, and/or competition between individuals keeping these two away from the dietary supplements you feed, etc), I would rec. bloodwork on these two individuals to try and establish why they are not utilizing their calcium. Hope this helps!
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Stacey