Thursday, June 9, 2011

The Mayor of Casterbridge's daughter: family resemblance close and far

Thomas Hardy's 1886 novel The Mayor of Casterbridge has a complex plot.

Casterbridge, imaginary Wessex village
The main protagonist, Michael Henchard, opens the story in a drunken state, in which he sells his wife and infant daughter for 5 pounds, to a mariner named Richard Newson, who had dropped casually by for a quaff.  Eighteen years later, having become the Mayor of Casterbridge, Henchard's former wife appears in town with their grown-up daughter, Elizabeth-Jane....or is she?  Henchard adopts her and restores his honor by re-marrying his wife and adopting E-J.  Yet, hearing that she may have died and been replaced by a daughter sired by the mariner, he observes her sleeping.
In sleep there come to the surface buried genealogical facts, ancestral curves, dead men's traits, which the mobility of daytime animation screens and overwhelms.  In the present statuesque repose of the young girl's countenance Richard Newson's was unmistakably reflected.  He could not endure the sight of her, and hastened away.
This is an interesting reflection of common views at the time.  Identical twins show that genes can have remarkable ability to generate even subtle aspects of a person's (or other species') traits.  This is often the subtle undercurrent behind genetic determinism.  Darwin knew that environmental effects could be important, but perceptively opined that a rare trait found in close relatives was probably a genetic (inherited) trait.  This was consistent with his theory of inheritance.  But what about atavistic traits, or 'ancestral curves' that seem to come back generations later?

Mendel gave us one explanation, recessiveness.  Truly atavistic traits like short tails in humans or toes in horses, are explained as developmental anomalies, but ordinary recessive traits are around and noted, but skip generations.  Darwin certainly knew of these, and in The Variation of Animals and Plants Under Domestication he tried valiantly but rather forcedly to explain them within his pangenesis theory of inheritance.

But what about things non-Mendelian that, as in Hardy's tale, seem to come back as vague shadows of the past?  These don't segregate in the fashion of classical dominant or recessive traits, though we are still taught that such traits are fundamental in genetics.  Instead, we see quantitative resemblances among relatives, that can be uncannily vague but clearly inherited.  Such traits, not appearing faithfully generation upon generation, clearly happen.

They are likely to be 'polygenic'--involve many different contributing genes.  Biology is rife with such traits, such as glucose levels, blood pressure, or stature in humans.  Relatives resemble each other due to the roughly additive contributions of countless loci.  You're expected to have a trait mid-way between that of your parents, unless environments play a major role.  And it has become the game (or, sometimes, shame) of GWAS studies to chase such traits and promise that the genes will be found and each person's future will be predictable from them.

But the vague family resemblances of complex, multivariable things like facial characteristics (or, for anthropologists, cranial size and shapes) have a rather unclear place in modern genetics.  We can see the resemblance but it is often difficult even to define just what it is.  It is not traits that segregate, even though people search for the genes 'for' them.  Yet they are not simple regressions of relatives' measurements, the way milk yield in cattle or stature in humans are.

The way such traits 'skip' generations presumably has to do with the effects of combinations of contributing alleles that, depending on what the other parent contributes in a given generation, are more or less easily recognized by our amazing image-processing software (our brains).  The more generations of removal, the fewer the contributing alleles that are still present in a parent.  The traits clearly are genetic, can occasionally seem to be very primitive ('atavistic') from eons past, yet they are not Mendelian the way green or yellow peas were for Mendel.  Yet these subtle aspects of relationships can be the stuff of evolution--the traits they affect change due to chance and natural selection, gradually molding the variation in the contributing genes.

The old quip is that if a child looks like the man of the house, it's genetic, but if it resembles the postman, it's environment.  To poor Henchard, one glimpse of Elizabeth-Jane's visage was devastating.  Had she resembled himself, he would have known it was genetics in action, and she was his own.  But he wasn't a modern man, so when he saw the resemblance to Newson, he wasn't coy enough to blame it on the environment!

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