The age-old question “nature versus nurture” has baffled brilliant minds for centuries. Are we the product of the DNA we inherit from our parents or the environment they produce for us? In favour of the later, English philosopher John Locke argued that people are born a “blank slate” shaped entirely by their upbringing and environment. In favour of the former, the famous geneticist Gregor Mendel is best known for discovering the laws governing heredity. Today, experts recognize the complex interplay multiple environmental and genetic factors have in shaping a single trait such as height, eye-color and even intelligence. How these factors are inherited remains an important point of debate. Here, I discuss heritability and how the Dutch famine put into question fundamental concepts of heredity.

The degree to which traits are caused by genetic factors is known as heritability and is a direct product of the DNA we inherit from our parents. Environmental factors such as diet and lifestyle exert their effects on our traits by creating chemical modifications on the backbone of DNA that are collectively known as epigenetic changes, but these are not inherited from one generation to the next. For changes in our DNA to be heritable and passed down to our children, they have to occur in our germline (sperm and egg cells) but this does not happen very often. It is far more likely that changes to our DNA are harmful rather than helpful so many protective mechanisms exist to ensure these changes do not occur in the germline.

The first sign of evidence that acquired epigenetic changes to our DNA may be inherited through our germline came in mid-1900s during the Dutch famine. In 1945, a German blockade cut off all food imports to the Western provinces of The Netherlands during World War II, starving millions of Dutch citizens in what was known as the Dutch Famine. Cohort studies followed up with pregnant women during the famine and found their children suffered health defects much later in life. The children were found to be far more susceptible to cardiovascular problems, obesity and diabetes later in life than children whose mothers did not experience famine. It was believed epigenetic changes acquired during the famine in the mothers were inherited by the children by a mechanism known as trans-generational epigenetic inheritance.

But it remains unclear if these trans-generational effects could be attributed to epigenetic inheritance for three reasons.

  1. Mammalian fertilization all epigenetic changes to be removed to create the stem cells that will go on to form every cell in the body. This reprogramming removes all epigenetic changes and the epigenome is rewritten from scratch and altered by the environment throughput the child’s lifespan.

  2. Studies referencing the Dutch famine fail to account for possible in-utero effects that starvation and malnutrition have on the developing fetus after fertilization. Acquired epigenetic changes may actually be attributed to maternal malnutrition during pregnancy instead of acquired epigenetic changes during the mother’s life being passed to the germline before fertilization.

  3. These studies are also confounded by non-genetic factors such as cultural and social inheritance that may over a lifetime recapitulate an epigenome that resembles that of our parents. Generational trauma is a well-recognized phenomenon whereby the stress from trauma experienced by a parent is also experienced by their children. However, it remains unclear to what degree generational trauma is inherited socially rather than by epigenetic inheritance.

For these three reasons, it is difficult to say if this phenomenon exists in humans. That’s not to say it does not, just that it is very difficult to test. This phenomenon, whereby epigenetic modifications acquired during the lifespan of an individual can be inherited by that individual’s child has huge implications for our understanding heredity and the “nature vs nurture” debate. If this phenomenon exists, acquired epigenetic changes as a product of stress or trauma experienced at any point in our lifetime would be inherited by our child. If this phenomenon does not exist, then we are in fact a “blank slate” for stress and trauma acquired during our parent’s lifespan.

References

https://www.nature.com/articles/s41467-018-05445-5
https://www.nytimes.com/2018/01/31/science/dutch-famine-genes.html