There is a code which, properly interpreted and implemented, serves as a blueprint for living things, and is required for all living, reproducing things. Like all good codes it has a very simple underlying structure, in this case only 4 letters. For humans, those letters are repeated in a sequence 3 billion base pairs long. Cunning.
This allows the transmission of many billion different messages. It is somewhat more complicated than that, because the code comes in 23 different physical bundles, and patches of particular sequences may be close to each other or further apart, which is another clue as to what may be going on. For example, a section of the code can act as a “GO TO” instruction, making the operating system jump forwards or backwards to another sub-routine.
No code is entirely unbreakable. It would appear that genetic codes conform to a “Great Chain of Being”, though not as Aristotle conceived it. That supreme thinker classified organisms in relation to a linear natural scale according to complexity of structure and function so that higher organisms showed greater vitality and ability to move. Evolution has done something similar, building up the ladder of life from very basic micro-organisms and then scaling up to greater integrative complexity. In that sense, living things are concatenations and elaborations of earlier solutions, which have been tweaked through natural selection, refinements being added to the code, usually making it a bit longer. Things which are useful survive and are replicated. We should not be too precious about the length of the code, since from that purely genetic perspective we are less complicated than the Paris Japonica plant and the marbled lungfish, the latter being elegant but not renown for its cultural achievements.
The genetic code includes some mistakes (though mercifully few fatal errors), dead ends, bits which are repeated, things which seemed a good idea at the time, fixes to problems which are no longer problems, and general code, miles and miles of general code which the evolutionary process has not dropped because, to anthropomorphize, who knows what the hell it does, but it seems to work, for the time being at least, and it could be fatal to mess around with it. At one time it was called “junk DNA” but now it is being treated more respectfully, because it probably sorts out problems which we have not yet even identified.
Again, like all good codes, some parts seem to be much more important than others. For example, most military outposts send messages about the weather, food supplies and other housekeeping matters. References to surprise attacks tend to be less frequent, and more circumspect. 99.9% of our genetic code may be no more than saying “I am a human, and I am digesting properly”. The remaining minority fragments are responsible for determining if that particular human is inclined to iambic pentameters, hallucinations, or to living quietly and abstemiously in a tranquil suburb. A small difference in the code can have big effects on behaviour. Letter for letter, most Bibles are the same, but the Barker and Lucas royal version of 1631 omitted the word “not” from the commandment they rendered as “Thou shall commit adultery”. I do not know to what extent this increased national adultery, but it would certainly have led to tension in small villages.
Most of the traits we are interested in, such as intelligence and personality, are probably influenced by very many genes of small effect. There is no single gene for intelligence, but there are shoals of them swimming in unison, though at present they are hard to find. Individual differences show up as personal variations in the genetic code: descants on the common melody that make us all human. These blips, called snips (single letter substitutions) give us our individual character and particular pattern of abilities. When the links in our code are very much like the links in a particular population then we are said to be in linkage equilibrium with that population. It shows that our ancient ancestral DNA comes from the same sources. It is similar to working out that a particular secret message comes from a particular foreign country, because it has the same general form. To hunt down an individual we are searching for the personal signal which stands out from the community noise. To search for an extended family we are looking for the common signal which distinguishes them from other distant extended families. Discriminant function analysis, cluster analysis, factor analysis, it is all the same difference. Find the central tendency, measure each deviation from that mean, and then plot out the discrepancies in whatever fashion makes most sense for the task in hand. Signal and noise, again and again.
I am sure that genetics would be more palatable to the general public if it was subjected to a thorough linguistic makeover. Perhaps it is only me, but when geneticists use “allele” instead of variant, and “loci” instead of location I tend to turn the page and read about something else. It’s code, guys, it’s code. I don’t need to see the wriggly bits.
You may have found this explanation far too simple, but if you can bear to delve into the wet wriggly stuff, then I am told that the following might be helpful:
Introduction to Quantitative Genetics, Fourth Edition [Paperback] Douglas S. Falconer and Trudy F.C. Mackay 1996. ISBN-13: 978-0582243026 Everyone seems to recommend this book for its explanations and worked examples, though it is not the most up to date. Researchers look back at it almost as a sacred text, not that scientists believe in such things.
Behavioral Genetics. 6th Edition. Robert Plomin, John C. DeFries, Valerie S. Knopik, Jenae M. Neiderhiser 2012. ISBN-13: 978-1429242158. Described as being the crossroads where psychology and genetics meet, it covers the traditional ground and the newer authors are giving this edition an up to the minute feel.
G is for Genes: The Impact of Genetics on Education and AchievementKathryn Asbury and Robert Plomin. 2013 ISBN-13: 978-1118482810
This new book, yet to be published but available for Kindle, has a far more specific focus on the genetics of scholastic achievement. It bills itself as a DNA to ABC text, but also covers the genetics of sporting ability, which makes a change from the familiar territory of IQ, motivation, special education and social status. It also propounds ideas, such as that continuity is genetic and change is environmental, genes are generalists and environments are specialists, and that the environments that matter most are those that are unique to individuals. It is written in a very readable format, probably intended for teachers. Depending on your background, it might be best to look at this one first.
From the point of view of psychology, all explanations compete on the same ground: Can the explanations lead to testable predictions? How well do those predictions match reality? Succinctly: Goodness of specification, goodness of fit.
As to genetics, just remember this: it is CODE just code. All you have to do is crack it.