Debunking Denialism

Defending science against the forces of irrationality.

Genetic Risk Factors and Parental Responsibility

The interaction of nature and circumstance is very close, and it is impossible to separate them with precision. Nurture acts before birth, during every stage of embryonic and pre-embryonic existence, causing the potential faculties at the time of birth to be in some degree the effect of nurture. We need not, however, be hypercritical about distinctions; we know that the bulk of the respective provinces of nature and nurture are totally different, although the frontier between them may be uncertain, and we are perfectly justified in attempting to appraise their relative importance.

- Sir Francis Galton, Inquiries into human faculty and its development (1883).

The nature versus nurture (or biology versus the environment) controversy has raged on for thousands of years. Modern science, however, has rejected this dichotomy as trivially false. It is not nature versus nurture, but nature through nurture. Both play essential roles in shaping organisms such as ourselves and they often interact with each other. However, as Galton remarked above, one could still discuss the relative merits of partial biological and environmental explanations. When people reduce the complex interaction of biology, psychology, biological and social environment to “mostly biology” or “mostly environment”, they are perpetually restraining humanity into the black-and-white cage that is nature versus nurture, despite paying lip service to modern science. Worse is that “mostly biology” is incorrectly interpreted as some form of genetic determinism, whereas “mostly environment” is erroneously conceived as the notion of the blank slate and the hail of vitriolic straw man arguments begin. The fact that some Internet commentators, journalists and other interested parties do not have sufficient scientific understanding, especially with regards to biology and psychology, makes it even more troubling. This, in turn, leads to a lot of misunderstandings about the science.

Clearly not the best setup for an intellectually productive discussion.

Generally speaking, genes work as risk factors. That is, the presence of a certain genetic background does not determine that the person will develop a certain physical or mental condition or set of personality traits. Rather, it should be understood as increasing the likelihood if all other factors remain constant. To be sure, there are situations when the presence of a certain genetic mutation will almost invariably result in a specific single-gene disorder (like Huntington’s disease), but these are exceptions, rather than the rule. For many conditions and traits, the environment are not a constant across everyone with a particular genetic background.

The environment often varies and it provides another layer of risk factors. Because of other factors, people exposed to the same environment may differ. To be sure, certain environmental influences, like getting decapitated, will invariably result in the same outcome (i .e. death), but this is again exceptions. Certain individuals with more genetic or biological risk factors may develop a condition after a certain environmental influence, whereas those without it may not develop the condition in question. A textbook example of this is the condition known as phenylketonuria (PKU).

The genetic risk factor for PKU is a mutated version of a gene coding for the enzyme known as phenylalanine hydroxylase that catalyze the hydroxylation of the amino acid phenylalanine to tyrosine. When this is non-functional, phenylalanine accumulates and is converted to phenylketones. This in turn causes mental retardation, brain damage and seizures. An incredibly successful treatment is a diet free of phenylalanine and monitoring of the blood levels of this amino acid. To get stuck in the nature versus nurture or even the “mostly biological” versus “mostly environmental” dichotomy would be unproductive, because it would miss the point entirely. Both biological and environmental factors are crucial and their interaction is often forgotten or ignored.

PKU is a fairly simple case. In most other cases, it is far more complex. On the biological side, you may have hundreds or thousands of single-nucleotide polymorphisms that contribute to the genetic background rather than just a single mutated gene. The gene products may also interact with each other, resulting in a stronger effect together than any of the did alone. On the environmental side, there may likewise be thousands of different environmental influences, from the very early periods of gestation. These two categories are also not strictly separate, as genes exist in the environment of other genes, and environment can regulate genes. Interactions between biology and environment is of utmost importance.

When it comes to mental conditions, a third layer of complexity is added called psychology. Psychological factors influencing mental conditions may themselves influence or be influenced by biology and environment, but may also have important effects on the development, maintenance and treatment of mental conditions in their own right.

It is important to understand that investigating and pointing to contributing explanations from biology is just a way to illuminate them in order to better understand the different influences and to find ways to prevent, modify or treat whatever it is being investigated. It is not an attempt to trivialize crucial contributing explanations from environment. There is rarely an “either-or”. There is only “together with”.

Equipped with this understanding, let us look at a popular misunderstanding: the notion that attributing a given condition to contributing factors from biology means giving up or dismissing parental responsibility. This issue can come up when discussing the heritability of mental conditions or personality traits. These are usually moderately large. Those that interpret this as an attack on the relevance of environment in influencing mental conditions and personality traits are uneasy with these results. However, this need not be the case. Heritability is the proportion of phenotypic variation that can be attributed to genetic variation in a population. Heritability is a property of populations and does not correspond to the degree to which the genetic background influenced the given trait in a given individual. For instance, the heritability of egg production is only about 0.1 (Brooker, 2012) . Does this mean that genes have almost nothing to do with egg production in poultry? Not at all. If it did, we would never have bothered with artificial selection for egg production in the first place. The thing that explains this peculiar fact is that artificial selection has reduced genetic variation in the population of poultry so that environmental variation accounts for more of the variation in egg production than does the genetic variation. A second lesson is that heritability estimates may vary from population to population. The heritability of height is much larger in Australia than in China (Lai, 2006). Is this because Australia has especially deterministic genes influencing height? Of course not. It is because in China, the variation in nutrients is a more important factor for the variation in height. I will discuss a review on the many myths surrounding heritability in a later article.

Conclusion

Heritability only tells us what proportion of population variation in a trait is due to the genetic variation in a population. A heritability value of, say, 0.6 does not mean that 60% of the trait of an individual is due to genes. This means that we can, without contradiction, accept the notion that various physical and mental conditions as well as personality traits have a moderately high heritability, while retaining the position that environmental influences (such as parenting) matters for the individual. This is because high heritability of a trait in a population does not imply that the trait is strongly determined by genetics. Also, the existence of genetic risk factors for a specific condition or trait should not make us dismiss parental responsibility. Quite the opposite, it shows the value of parenting (and environment at large) in order to reduce environmental risk factors that may, together with any genetic risk factors in the individual, trigger or influence the condition.

References and Further Reading

Brooker, R. J. (2012). Genetics: Analysis and Principles. New York: McGraw-Hill, Fourth edition.
Lai, C-Q. (2006). How much of human height is genetic and how much is due to nutrition? Scientific American. Accessed: 2012-06-02.

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