More info about this myth

It is easy to understand how this myth came into being. How often has someone told you that you “have the gene for brown eyes”, or maybe “you have the tall gene!”, but this is an oversimplification of inheritance.

Estimates suggest that humans have somewhere between 20,000 and 25,000 genes, which seems like a lot, but if each one had only one function, how could we possibly have enough genes to code for our appearance (hair colour, eye shape, skin tone), physical traits (whether we like the taste of cilantro, if we are right or left handed) abilities (athleticism, if we can digest milk, producing vitamin D) and the thousands of other things our DNA codes for? While the answer is simple, it is also complicated; most of our genes have more than one function.

When a gene has more than one function this is called “pleiotropy”. This term is derived from two Greek words; “pleio”, meaning “many”, and “tropic”, meaning “affecting”. As described in this video, an example of pleiotropy is that of the domestic feline. Cats with white fur have an autosomal dominant gene, which is given the shorthand label “W” (short for “white”). When cats have this W gene they always have white fur, which is an example of “complete penetrance” of a gene. Cats with the W gene often have blue eyes, and are deaf, but not always. This is an example of “incomplete penetrance” of a gene. So, the W gene always causes white fur, and often causes blue eyes and deafness. This shows that the blue eye colour and deafness of cats is influenced by more than one gene (this is called polygenetic inheritance).

White cats make up roughly 5% of the domestic cat population. Of these white cats roughly 40% have at least one blue eye. Those with two blue eyes have around an 80% of being deaf, and those with one blue eye have about 40% chance of being deaf. Interestingly, those cats with only one blue eye are likely to be deaf in the ear on the same side!

So how does this work? Well, the presence of W gene in cats suppresses the function of pigment producing cells called melanocytes. Because these cells do not produce pigment, the fur of these cats is white. The pigment in the cat’s eyes can also be affected, producing blue eyes instead of yellow. Those two are easy to see a connection with, but hearing? The cochlear in the ear of mammals includes a band of the melanocyte cells, which helps with the transmission of electrical signals to the brain from the ear. If these cells are affected, the sound waves enter the ear of the cat, vibrates the hair cells in the ear, but no message is passed onto the brain. Researchers are able to observe this by performing a brainstem auditory evoked response (BAER) test.

Pleiotropy does not just occur in white cats though, there are many other examples in both the plant and animal kingdoms, and not all pleiotropic genes result in negative effects. You may like to look at these slides from the Khan Academy to learn a bit more about pleiotropy.



Educators: For an additional free resource to use with your students you may like to investigate the ‘Alien Babies’ activity, created by our Geneskool team. You can find this resource here.