Linked Genes: What are the implications?
In the 1850s, Darwin predicted the very thing that continues to fascinate artists today as they explore the ethics behind genetic manipulation. Darwin writes, “If man goes on selecting, and thus augmenting, any peculiarity, he will almost certainly modify unintentionally other parts of the structure, owing to the mysterious laws of correlation.” Prior to this warning, Darwin lists a number of links he has observed: hairless dogs have imperfect teeth; pigeons with short beaks have small feet, and those with long beaks large feet. He also notes: “Some instances of correlation are quite whimsical: thus cats which are entirely white and have blue eyes are generally deaf…”
Are these connections old wives’ tales, coincidences, or genetically sound conclusions? I decided to research the claim that solid white, blue-eyed cats tend to be deaf. Could something as seemingly superficial as color really be linked to a disability? The answer is yes – Darwin was correct.
My lay-science person interpretation of the connection is as follows: In cats, the same stem cells produce 1) a layer of tissue in the eye—the tapetum lucidum 2) pigment cells and 3) a cell layer in the inner ear. (Because there are several genes that cause identical physical appearances—all-white coat and blue eyes, in this case—not all of these cats will be deaf. It’s the genotype – not the phenotype!) However, if the blue eyes of a white cat are the result of a deficiency in tapetum, the cat will also lack an important cell layer in the inner ear – which allows it to hear. Deafness, as you’ve probably already guessed, is the result of the absence of that cell layer. (Source: “All White Cats, Eye Color, and Deafness,” Sarah Hartwell)
Of white cats with at least one blue eye, an estimated 60 – 80% will be deaf. If you need more proof than that, take the example of the white cat with two differently colored eyes. In many cats with one green eye (or brown eye, etc.) and one blue eye, the ear on the side of the green eye will have hearing while the ear on the side of the blye eye will be deaf.
So then, Darwin was on to something (no surprise there). The connections between genes seem infinite and sometimes unpredictable.
This raises the question: Will we ever be able to map every connection between genes? And, unless we map every connection among linked genes, can we ever really know what else we’re going to change when we genetically alter something?