Another non-intuitive but common behavior of genes is that one gene can have different but apparently unrelated effects. This is somewhat the opposite of different genes having the same effect (discussed in my previous blog). Making it even more confusing, the same condition could have both at work!
One gene may create a specific appearance but can also cause issues that would not necessarily appear to be related. In scientific terms the word pleiotropy is used to refer to the phenomenon "when one allele or pair of alleles has different effects - particularly when these effects appear unrelated."
Many examples of this phenomenon exist in the dog world. With on-going research, more will no-doubt be identified.
The gene causing Merle color pattern increases the chance for deafness and eye defects. Certain "white" dogs also have increased risk of deafness related to being white. In other breeds, very white appearing dogs (due to other genes) and dogs with albinism have no associated risk for hearing loss. In the case of the Merle and "white piebald" associations the effect begins on cells derived from the neural crest affecting both pigment, auditory and potentially other neural-derived cells, but not all Merles, double merles and ...