Friday, September 16th
Some questions arise about what’s a shared experience and what’s a unique experience. “home” environment is shorthand—obviously, if something unique happens to one individual and that unique thing happened to occur inside the home, then it’s a unique experience. Likewise, as we pointed out last time, if siblings share a peer group and outside the home experiences, then these are not unique experiences for them.
Part of the difficulty comes from applying information about twins to non-related individuals in different environments…the source of heritability information and its applicability occurs in different places.
Our twin and adoption studies tell us that shared environments aren’t what make siblings alike…so they can’t be what make individuals growing up in different environments different.
So what does make individuals from different environments different, other than their genes? It’s the non-shared environment that must be the source of the variability. The things that happened to A3 but not A1 are what make A3 different from B1 (or B2 for that matter). How do we know? Because if the shared environment (the things that happen to A1, A2, and A3) are what made A3 different from B1, we’d expect that A1, A2, and A3 would be more similar to one another than we’d predict based on their genetic similarity. But we know that siblings are no more alike than we’d expect them to be because of their genes.
Again, there’s nothing special about the fact that it’s the “home” environment…it’s
simply that we can tell that the things that happen to all individuals growing
up in the same household is not the source of their similarity—all of their
similarity can be explained by their genes.
If the shared environment isn’t responsible for the similarity among
siblings, then it can’t be the reason individuals differ.
Remember underlying all of this is the observation that concordance of MZA=MZT, and for most personality traits, concordance of MZT is roughly equal to heritability. There is no appreciable amount of similarity among them that’s due to shared environment.
Ok…so continuing where we left off…all of our discussion thus far has been for personality traits…roughly 50% heritable, small percentage of variability is due to home environment, rest comes from unique experiences.
Pinker discusses the theory that these unique
outside-the-home non-shared experiences are responsible for the variations in
our personalities but he also disagrees with the assessment in one important
respect. He feels that although these
peer group experiences can determine aspects of our behavior…our
socialization—how we determine what is the appropriate
behavior in a given set of circumstances—he feels that the peer group is
unable to influence stable aspects of personality. So if differences in personality are due
partly to genetics and only marginally to the upbringing that you share with
your siblings, then where does the rest of the variation come from? And here is where he invokes chance—random
effects of the environment in shaping who we are. No-one explains it as well as Pinker does, so
I would like to move on from here and stress the point that there are important
issues from the Chapter 19 reading that
I can’t explore as in depth as I would like. So I would like you to be familiar with the
points that are made in this reading.
Before we move on, I thought I’d make a plug for other important behavioral traits that aren’t heritable: sense of humor, food preferences, social and political attitudes (although overall conservativeness does show heritability), religion (although not religiousity).
Now that we’ve introduced the idea that genes in some way influence behavior, we should probably spend a few minutes discussing what a behavior-relevant gene might look like.
Genes relevant to behavior: we have already discussed the idea of genes as being “nature” and our environments as being “nurture”. Let’s take a closer look at what genes are, and how they influence behavior. Each (most) of our cells in our bodies contains a nucleus--inside the nucleus, is the genetic material--the chromosomes, which are twisted pairs (23 of these pairs in each cell) of DNA. DNA is composed of matched pairs of bases--A,C,T,G. The particular sequence of DNA is unique to each individual, and is represented in each of your body’s cells. DNA gets “transcribed” into mRNA--its called mRNA because its a messenger, it contains a message--depending on what the original DNA that it was transcribed from. Now, the mRNA gets translated into proteins, in a process known as protein synthesis...each 3 letter sequence in the mRNA determines a particular amino acid…the amino acids get strung together to form proteins. SO, you see all that the DNA does is tell each of your body’s cells which proteins to produce. A structural gene is a coding region of DNA that specifies a particular protein. Regulatory genes are regions of the DNA that turn on or off gene expression (i.e., protein synthesis).
WHY IS THIS PROCESS IMPORTANT???
This process is important because proteins are the building blocks of our cells. They are integral parts of the cell membranes, of the internal structure of the cells, they are the enzymes which make specific reactions take place within cells, they are specialized regions on cell surfaces called receptors, they are the chemical messengers of the nervous system. So, which proteins are expressed in which cells--and this is important, only parts of the genetic message are expressed in each cell-- determines what that cell’s function is. OK--so now we know what proteins are, and how they are synthesized from chromosomes which are made up of DNA--genes are simply portions of the DNA--the section of chromosome that is responsible for the production of a single protein is called a gene (particularly a structural gene). There are also specialized regions within the DNA itself, which essentially determines whether those genes will be used in a given cell--this is how even though each cell contains all of the genes, we have cells that are as different as neurons and liver cells.
This should also stress that a GENE isn’t a specific behavior producer…all that a gene can do is to produce a protein, which may have a biologically relevant function, OR may turn on or off other genes Genes can produce changes in neurochemistry and neurophysiology which can increase or decrease the tendencies toward particular types of behavioral responses. The effects of genes on behavior are always probabilistic.
I’ve got 3 examples of behavior relevant genes that I want to discuss because each I think illustrates a different point:
As we discussed earlier this week in our discussions of heritability of personality, some amount of variability in the subscales of the Big 5 personality inventory is accounted for by genetic variation. For most of these traits, we have no idea what genes are involved (and twin studies are essentially useless in this regard)—it’s likely to be many genes, but recent research is actually beginning to shed some light on the particular genes that may be involved. Take the gene for BDNF, on Chromosome 11. Gene that everyone has, in most individuals the 192nd spot on the gene is a G, in others it’s an A. That “variant” causes a slightly different protein to be built—one with a methionine in the 66th position instead of a valine. Since you have 2 copies of this gene—one from each of your parents, you either have BDNF’s with only methionine, only valine, or some with methionine some with valine: met-met; val-val; met-val. Val-val’s are most neurotic, val-met’s significantly less so, least neurotic are the met-met’s. SO having this gene variant is associated with being noticeably less neurotic. Not the ONLY gene that is likely to be associated with this personality trait—only accounts for 4% of the variability in neuroticism among the people tested in the study—but it is fairly specific for this trait because the rest of the traits were not associated with this gene variant. And of course, it may not be the case in all people, just those that were studied in the sample.