The Language Learning Disability Gene
First, results reported in an April 2010 conference paper regarding a language learning disability gene (phrase structure and bullet order reorganized in bullet result section for clarity):
[Morten Christiansen] studied 159 8th graders. . . 100 were normal language learners; 59 of them had language impairment, although their non-verbal IQs were similar to those of the other group.
DNA tests looked at one part of the FoxP2 gene. A typical gene has a molecule symbolized by T; atypical genes can be symbolized by C. As each person’s genome gets a gene from each parent, the genotypes can be classed as TT, TC, or CC. The DNA tests found that:
•70% of TT students had normal language, 30% were language impaired;
•51% of TC students had normal language; 49% were language impaired.
•30% of CC students were normal; 70% had impaired language;
This data is pretty clear. Language impairment does not come only from an abornormality in FoxP2, but much of it does.
Christiansen also conducted a sequential learning study with the students and found that the CC students had a much tougher time learning the task than the TT or CT. Thus, the FoxP2 gene that is associated with language learning problems is also associated with difficulties in a non-verbal, sequential learning task.
The impact of this gene is even greater when the heritability of verbal test ability, without regard to any specific cause of that heritability, is only about 0.60. If 0.40 of the variation in performance on verbal tests (and hence, presumably language ability) is not genetic, then this single gene may be accounting for a very large share of all genetic impacts on language ability given the numbers in this study.
The Bullying Impact Gene
Then there is the case of the gene associated with vulnerability to emotional harm from bullying:
[B]ullied kids who happen to have inherited one form of a stress-related gene develop the most emotional problems.
Symptoms of anxiety, depression and social withdrawal appeared most often in regularly bullied kids who possessed two copies of a short version of the 5-HTT gene . . . . One-third of bullied children who had two shorter copies of the gene displayed emotional problems severe enough to merit mental health treatment. . . That figure fell to 29 percent for regularly bullied kids carrying one short copy of the gene and 15 percent for those with two long copies. . . . In cases where each twin carried two short copies of the 5-HTT gene but only one got repeatedly bullied, emotional difficulties were observed only in the bullied twin[.]
Citations (re bullying gene):
* Sen, S., et al. 2010. A Prospective Cohort Study Investigating Factors Associated With Depression During Medical Internship. published online April 5, Arch. Gen. Psychiatry. doi:10.1001/archgenpsychiatry.2010.41.
* Sugden, K., et al. 2010. Serotonin Transporter Gene Moderates the Development of Emotional Problems Among Children Following Bullying Victimization. published online May 14, Journal of the American Academy of Child & Adolescent Psychiatry. doi:10.1016/j.jaac.2010.01.024.
* Benjet, C., et al. 2010. 5-HTTLPR moderates the effect of relational peer victimization on depressive symptoms in adolescent girls. Journal of Child Psychology and Psychiatry 51 (2):173. doi:10.1111/j.1469-7610.2009.02149.
It is really rather stunning that such simple, easily tested for genetic factors can have such a profound influence in people's lives that we haven't normally classified as hereditary or medical issues (the "bad husband gene" is another one of this stripe).
The good news is that these studies make diagnosising risk factors for these, and no doubt other socially important traits, is no longer pie-in-sky science fiction. Once you've found a particular gene that you want to type, tests for it can be mass produced and made very cheap. It is even possible to cheaply mass produce devices that can test for multiple genes.
A pediatrician appointment in 2020 may routinely include putting a few drops of blood into a handheld device you can hold in your hand that costs under $200 and learning the risks that a child faces on half a dozen of these factors. For good or for ill, genetic testing before marriage may take on a whole new development as these genes are discovered.
The bad news is that knowing someone is at risk doesn't necessarily mean that there is much that can be done to treat the challenges caused by particular genes. It is impossible to avoid using language in our society. Responding badly to stress is the mental health equivalent of an immune system disorder with no known treatment. An inability to form lasting couple relationships in your personal life is a pretty awful curse to grow up knowing about.
Indeed, knowing that someone has a genetic predisposition to a problem may discourage efforts to address it. Yet about 30% of students with a genetic predisposition to having language related learning disabilities don't appear to suffer from them, and extra help in school helps many kids overcome them. Knowing a child's genes may lead to a culture of low expectations in their educators.
One also suspects that traits so common in the population can't be exclusively negative. A large part of the natural selection process in people today takes place in the process of finding someone with whom to have children, and has had this character for many generations. If these traits were too harmful and had no upsides, they would be rare, not commonplace.
Then again, it may be that these traits are recently obsolete and will soon become much more rare. They are more serious impairments now than they used to be not so many generations ago. Our society is rapidly becoming one that is universally literate, has a higher population density, allows prospective spouses rather than parents to arrange marriages, and permits women control their fertility, in and out of marriage, to a much greater extent than in prior era. These changes in our society may lead to rapid selection against traits that impair functionality in this new society.