Billiard-ball genetics

2 minute read

I picked up a copy of Julian Huxley’s Evolution: The Modern Synthesis this week at a book sale. It’s funny – the book was a review copy and bears the following bookplate:

To The Literary Editor:
Direct quotation in reviews is limited to 500 words or less unless special permission is given.

Well, I hope that the statute of limitations on the bookplate has passed, because I’m going to quote a lot more than 500 words out of this over the next few weeks.

I’ll start with a passage from the first chapter. It brings to mind Mayr’s famous comment about “bean bag genetics,” but I find Huxley’s approach at once more sympathetic and insightful about the nature of inheritance opposed to the way scientists describe inheritance:

In the early days of Mendelian research, phrases such as "in fowls, the character rose-comb is inherited as a Mendelian dominant" were current. So long as such phrases are recognized as mere convenient shorthand, they are harmless; but when they are taken to imply the actual genetic transmission of the characters, they are wholly incorrect.
Even as shorthand, they may mislead. To say that rose-comb is inherited as a dominant, even if we know that we mean the genetic factor for rose-comb, is likely to lead to what I may call the one-to-one or billiard-ball view of genetics. There are assumed to be a large number of characters in the organism, each one represented in a more or less invariable way by a particular factor or gene, or a combination of a few factors. This crude particulate view is a mere restatement of the preformation theory of development: granted the rose-comb factor, the rose-comb character, nice and clear-cut, will always appear. The rose-comb factor, it is true, is not regarded as a sub-microscopic replica of the actual rose-comb, but is taken to represent it by some form of unanalysed but inevitable correspondence.
The fallacy in this view is again revealed by the use of the difference method. In asserting that rose-comb is a dominant character, we are merely stating in a too abbreviated form the results of experiments to determine what constitutes the difference between fowls with rose-combs and fowls with single combs. In reality what is inherited as a Mendelian dominant is the gene in the rose-combed stock which differentiates it from the single-combed stock: we have no right to assert anything more as a result of our experiments than the existence of such a differential factor (Huxley 1943:19).

I try to emphasize this point whenever I introduce genetics: we know about inheritance because of our observations on organisms, not because we have traced the molecular effects of every gene. As when we interpret sounds into language, we depend on contrasts to see the effects of alleles. I am always amazed when we learn something new about these molecular mechanisms underlying observable phenotypes, because they manifest in so many different ways.

P. S. Yes, after all that, you deserve a picture of a rose-comb. But although Flickr has many, all are copyrighted, none Creative Commons. So, I won’t republish, but here’s a link to my favorite.