A Serendipitous Discovery
I remember the first time I learned about epigenetics. It was during my undergraduate degree in Molecular Genetics. However, I did not learn about it from lecture, but rather while watching an episode of NOVA, a PBS documentary program that focuses on science.
They were talking about a study that discovered a link between a population of people in Ireland who had an increased prevalence of Type II diabetes, high blood pressure and obesity. The Irish potato famine occurred in the 1800s and saw a blithe attack a main food source – potato crops. Here’s a link that discusses the potato famine in more detail – Irish potato famine. The descendants of women who lived through the potato famine gave way to grandchildren who made up that group of people with a higher prevalence of Type II diabetes, high blood pressure and obesity.
How could this be? The group with the higher prevalence of those conditions listed had come from grandmothers who survived the potato famine. The medical conditions seemed to skip a generation. There was no known mechanism that could account for this occurrence. It seemed to fly in the face of the central dogma and the current understanding of evolution. The central dogma states that DNA is transcribed into RNA and then translated into protein. Our understanding of evolution was survival of the fittest – that those who survived to reproduce would have their genome passed on to their offspring.
So what happened? The researchers surmised an incredible and complex mechanism that explained how these conditions had somehow skipped a generation. It goes like this.
Environmental Exposure and Epigenetics
It all starts with a women who is pregnant. The embryo is female. Women are born with all of their ova or eggs developed. These eggs will be released, one by one, at puberty and the onset of the menstrual cycle, for the rest of the woman’s reproductive years . Men, on the other hand, are not born with their full complement of gametes or sperm. For men, their sperm is produced basically on demand at the onset of puberty.
So, the female fetuses’ eggs are being developed in utero. The woman carrying the fetus is exposed to environmental factors (in this case famine) which results in changes to the DNA of the embryo, including the DNA found in the eggs of the developing fetus.
What changes occur in the DNA of the fetus and her developing ova or eggs? The DNA of both the fetus and her eggs is marked with chemicals (based on, in this case, environmental factors) that determine what genes will be expressed and what genes will not be expressed.
The fetus is then born with her full complement of eggs. She reaches puberty and an egg is released at each menstrual cycle. One day, she becomes pregnant and gives birth to a baby. That baby grew from an egg that had been epigentically marked while in utero.
This is how the environmental factors a grandmother experiences can be passed on to not her children but her grandchildren’s genome. These genetic markings are termed the epigenome. The epigenome adds another layer of complexity to genetics and evolution. Next, I’ll discuss behavior and the epigenome.