Epigenetics: effects of environment on genes

There has been a pervasive thought in both biology and medicine that humans are limited by their genes. It's true that traits like height and eye color are dictated solely by the genes you inherit. If you want to change your eye color, for instance, contact lenses are the only way to do this — you can't manipulate these traits.

Diseases also have a genetic component. So, are we locked in by our by genes, as far as disease goes? Not necessarily. Most chronic diseases are influenced by a combination of genes and environment. This means that your family history of cancer or diabetes, for instance, does not necessarily mean that you are highly likely to get the disease. Let me explain.

Epigenetics is a burgeoning new field with a potentially powerful impact on preventing and treating chronic diseases. Literally, it means "above the gene." In other words, epigenetics regulates gene expression (the phenotype), or the turning on and off of genes. Epigenetics can have beneficial or detrimental effects, depending on the environment. It does this through transcription factors — proteins that bind to genes and determine whether they are expressed or suppressed. There are at least 2,000 transcription factors (Science 2002;295:813-818). Examples of some of the more researched transcription factors include NFkB, increasing oxidation and inflammation; p53, a tumor suppressor; and NRF-2, antioxidant response. However, epigenetics does not alter the DNA sequencing of the gene itself. This differentiates it from gene therapy (in which an attempt is made to change the genes) which is a more complicated process that has thus far eluded medicine, with a few exceptions.

Bear with me here, but this is much like the cat that ate the spider who swallowed the fly. Environmental factors, such as diet, toxins, drugs and exercise affect which transcription factors are up or down regulated and then, in turn, the genes that are turned on or off. For instance, vitamin D may have an effect on over 200 genes (Am J Clin Nutr. 2007 Sep;86(3):645-51).

To date, cancer is the chronic disease most extensively studied. Cells can be transformed into cancer cells due to down regulation of tumor suppression genes and expression of oncogenes, pro-cancer genes. The opposite is also true. Epigenetics can cause tumors to either proliferate or be suppressed, depending on environmental influences.

Diet plays a central role in determining whether cancer develops.

Dominant isn't Always Common | QUEST Community Science Blog - KQED

One of the first things we’re taught in genetics is that some traits are dominant and others are recessive. And that the dominant traits trump the recessive ones.

So brown eyes trump blue eyes. And red hair is always trumped by other hair colors. And so on.

From this, people often jump to the conclusion that the dominant trait is also the most common one. This isn’t always the case and there is no reason it should be.

Whether or not a trait is common has to do with how many copies of that gene version (or allele) are in the population. It has little or nothing to do with whether the trait is dominant or recessive.

Let’s take eye color as an example. The decision on whether to have brown eyes or not is pretty much controlled by a single gene, OCA2 .

We can think of OCA2 as having two versions, brown and not-brown. The brown allele of OCA2 is dominant over the not-brown allele.

Nearly everyone in most of Africa has brown eyes. This isn’t because brown eyes are dominant over blue and green. Instead, it is because there are mostly brown alleles of OCA2 in the African population.

Northern Europe is a different story. In some parts of the continent, over 80% of the population has lighter colored eyes. Here the not-brown allele is more common even though it is recessive.

Now this allele isn’t exclusive, there are still brown-eyed folks in northern Europe. So why don’t their brown eyes dominate over time? Because in populations, dominant isn’t dominant over other people’s recessive gene versions. Your brown eyes can’t affect my kids’ eye color unless we get married.

Let’s do a thought experiment to make this clearer. To simplify things we’ll call brown eyes B and not-brown eyes b.

Remember, you will have brown eyes if you are BB or Bb and blue or green if you are bb. This is because brown (B) is dominant over blue and green (b).

Imagine we start out with eleven bb people and one Bb person. The Bb person has 4 kids with one of the bb folks and each bb couple also has 4 kids.

Using regular old Mendelian genetics, we'll have 20 bb people from our 5 bb couples and 2 Bb and 2 bb from our mixed couple. This is 2 people with brown eyes and 22 people with blue or green. The same ratio as we started with. Brown did not become more common.

Now these folks all pair up randomly and have 4 kids each. Since we aren't going to allow incest, the Bb folks will find a bb for a mate. If they have 4 kids each, then we have 44 bb and 4 Bb. Again the same eleven blue to one brown ratio. Whether an allele is dominant or not does not affect how common a trait is.

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