Escher 6 1/2 weeks

He never had a single hair on him in his life, but as he's hitting "puberty," Escher has grown coarse hair. Heh. His siblings grew soft fuzz and then went bald, as hairless rats usually do. I believe Escher's odd coat is due to the presence of rex genes. 

(Hairlessness is epistatic to other coat types, but other loci can have subtle effects.)