You might recall from high school biology a scientist by the name of Jean-Baptiste Lamarck. He proposed a mechanism of evolution in which organisms pass on traits acquired during their lifetimes to their offspring. The textbook example is a proposed mechanism of giraffe evolution: If a giraffe stretches its neck to reach higher leaves on a tree, the giraffe would pass on a slightly longer neck to its offspring.
Lamarck's proposed mechanism of evolution was tested by August Weismann. He cut off the tails of mice and bred them. If Lamarck was correct, then the next generation of mice should be born without tails. Alas, the offspring had tails. Lamarck's theory therefore died and remained largely forgotten for over 100 years.
However, some scientists believe that new data may at least partially resurrect Lamarckian thinking. This recent resurgence is due to a new field called epigenetics. Unlike regular genetics, which studies changes in the sequence of the DNA letters (A, T, C, and G) that make up our genes, epigenetics examines small chemical tags placed on those letters. Environmental factors play an enormous role in determining where and when the tags are placed. This is a big deal because these chemical tags help determine whether or not a gene is turned "on" or "off." In other words, the environment can influence the presence of epigenetic tags, which in turn can influence gene expression.
That finding is certainly intriguing, but it isn't revolutionary. We've long known that the environment affects gene expression.
But, what is potentially revolutionary is the discovery that these epigenetic tags, in some organisms, can be passed on to the next generation. That means that environmental factors may not only affect gene expression in parents, but in their yet-to-be-born children (and possibly grandchildren), as well.
Yikes. Does that mean Lamarck was right? That question was addressed by Edith Heard and Robert Martienssen in a detailed review in the journal Cell.
Of particular concern is the idea that mammalian health can be affected by epigenetic tags received from parents or grandparents. For example, one group reported that pre-diabetic mice have different epigenetic tag patterns in their sperm and that their offspring have a higher chance of contracting diabetes. (Virginia Hughes has written an excellent article summarizing this and other related epigenetic studies.) A flurry of other biomedical and epidemiological research has strongly hinted that a susceptibility to obesity, diabetes, and heart disease can be passed on through epigenetic tags.
However, Heard & Martienssen are not convinced. In their Cell review, they admit that epigenetic inheritance has been demonstrated in plants and worms. But, mammals are completely different beasts, so to speak. Mammals go through two rounds of epigenetic "reprogramming" -- once after fertilization and again during the formation of gametes (sex cells) -- in which most of the chemical tags are wiped clean.
They insist that characteristics many researchers assume to be the result of epigenetic inheritance are actually caused by something else. The authors list four possibilities: Undetected mutations in the letters of the DNA sequence, behavioral changes (which themselves can trigger epigenetic tags), alterations in the microbiome, or transmission of metabolites from one generation to the next. The authors claim that most epigenetic research, particularly when it involves human health, fails to eliminate these possibilities.
It is true that environmental factors can influence epigenetic tags in children and developing fetuses in utero. What is far less clear, however, is whether or not these modifications truly are passed on to multiple generations. Even if we assume that epigenetic tags can be transmitted to children or even grandchildren, it is very unlikely that they are passed on to great-grandchildren and subsequent generations. The mammalian epigenetic "reprogramming" mechanisms are simply too robust.
Therefore, be very skeptical of studies which claim to have detected health effects due to epigenetic inheritance. The hype may soon fade, and the concept of Lamarckian evolution may once again return to the grave.
Source: Edith Heard and Robert Martienssen. "Transgenerational Epigenetic Inheritance: Myths and Mechanisms." Cell 157 (1): 95–109. (2014). DOI: http://dx.doi.org/10.1016/j.cell.2014.02.045