Dwarfism
While uncommon, dwarfism in chickens does exist. Three types of dwarfism have been identified in chickens:
All three are caused by a mutated or recessive gene. Thyrogenous dwarfism, caused by the (td) gene, results in stunted birds that do not reach sexual maturity.
Autosomal dwarfism, due to the (Adw) gene, yields growth-delayed birds approximately 30 perecnt smaller than their counterparts.
Sex-linked dwarfism, connected to the recessive (dw) gene on the Z sex chromosome, manifests every now and then in purebred flocks and results in miniature, proportionate versions of normal birds. These tiny chickens breed and lay just like large fowl, albeit on a smaller scale.
Since TJ’s egg was perfectly normal in size and shape, and since his body is proportional and not stunted, my conclusion is that his size is due to sex-linked dwarfism and the (dw) gene, especially given that he comes from a long line of purebred Orpingtons.
We have nothing to worry about, health wise. He might be tiny, but he’s as active and voracious as his siblings and definitely here for the long run.