Download Understanding Heritability and Epigenetics

Understanding Heritability and Epigenetics
When you think "heritability" or "heritable", replace it in your mind with "varied" or "differences." Heritability
and inherited are nearly the opposite in meaning, and the idea of heritability is more about the varied differences
BETWEEN and among groups rather than "passing down genes." Heritability statistics resulted from the work of
Francis Galton, who was Charles Darwin's cousin. Heritability is a group statistic that makes no sense when
applied to one person. Heritability is the extent to which differences in the appearance of a trait across several
people can be accounted for by differences in their genes. Heritability does not reflect the extent to which traits are
passed down from parent to offspring .
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Identical twins have a heritability of 0, since any variation in their behavior cannot be accounted for by
genetic differences.
As the environment gets more similar for individuals of very different heritabilities, heritability increases.
This is because as the environment gets more similar, they become less of a source of variation for the
individuals.
The heritability of a group of individuals with relatively similar heredities in very different environments is
relatively low. This is because as the heredities become more similar, they are less of a source of variation
for the individuals in the group.
Epigenetics is more about understanding how the environment affects genes and, therefore, behaviors. It is the
study of how variation in inherited traits can originate through means other than variations in DNA. Epigenetics
refers to the epigenome. The prefix “epi” means above, while “genome” refers to all of an individual’s genetic
information. Thus, the epigenome is information about us that is stored outside of our DNA — just outside of it, as
it happens. Specifically, special chemicals called tags can become attached to the nucleotides of our DNA or to our
histone proteins, and, depending on the nature of these tags, specific genes can either be silenced (prevented from
being expressed as protein) or pushed to become more active (so that the abundance of a particular protein in our
cells will rise).
Tags that shut down, or silence, the expression of a gene include methylgroups. For example, the methylation
(attachment of methyl groups) of tumor suppressor genes in cells infected with Epstein-Barr virus inactivates those
genes, thereby promoting tumor formation (Kaneda et al., 2012). A second major type of tag, called an acetyl
group, may be added to our histone proteins. The addition of acetyl tags (acetylation) weakens DNA-histone
binding, making the DNA at that location more available to the proteins that drive transcription (Sapolsky, 2004).
For example, garlic has been shown to increase the acetylation (and therefore the activity) of anti-cancer genes
(Druesne-Pecollo and Latino-Martel, 2011).
Beyond the use of medicines, individuals may be able to exert direct control over their epigenome simply by
modifying their diet or exposure to certain chemicals. For example, bisphenol A (BPA), a compound used to make
certain plastics, was shown to decrease the methylation of a specific gene in mice, while a supplement of folic acid
was shown to prevent that change (Dolinoy, Huang, and Jirtle, 2007).
One major surprise of epigenetics is “transgenerational transmission.” A notable demonstration of this concept
came from a study of mice raised in an “enriched environment,” meaning one that included high levels of exposure
to novel objects (e.g., cardboard boxes and pet toys), interactions with other animals and opportunities for exercise
(Arai et al., 2009). Compared to control mice, the enriched mice exhibited enhanced long-term potentiation
(LTP), and the offspring of these animals exhibited the same enhancement in LTP — even if they had not been
raised in an enriched environment. While this effect was relatively short lived (it did not transfer to the second
generation), it showed that changes to the epigenome can be transmitted to offspring.