Epigenetic inheritance
Epigenetic inheritance adds another dimension to the modern picture of evolution. The genome changes slowly, through the processes of random mutation and natural selection. It takes many generations for a genetic trait to become common in a population. The epigenome, on the other hand, can change rapidly in response to signals from the environment. And epigenetic changes can happen in many individuals at once. Through epigenetic inheritance, some of the experiences of the parents may pass to future generations. At the same time, the epigenome remains flexible as environmental conditions continue to change. Epigenetic inheritance may allow an organism to continually adjust its gene expression to fit its environment – without changing its DNA code.
Reprogramming of genes
Reprogramming is important because eggs and sperm develop from specialized cells with stable gene expression profiles. In other words, their genetic information is marked with epigenetic tags. Before the new organism can grow into a healthy embryo, the epigenetic tags must be erased.
At certain times during development (the timing varies among species), specialized cellular machinery scours the genome and erases its epigenetic tags in order to return the cells to a genetic “blank slate.” Yet, for a small minority of genes, epigenetic tags make it through this process and pass unchanged from parent to offspring.
Nutrients and genes
The nutrients we extract from food enter metabolic pathways where they are manipulated, modified, and molded into molecules the body can use. One such pathway is responsible for making methyl groups – important epigenetic tags that silence genes.
Familiar nutrients like folic acid, B vitamins, and SAM-e (S-Adenosyl methionine, a popular over-the-counter supplement) are key components of this methyl-making pathway. Diets high in these methyl-donating nutrients can rapidly alter gene expression, especially during early development when the epigenome is first being established.
How are traits passed from one generation to the next?
1.Some of our genes come only from Mom.
Mendel believed that parents contribute equal numbers of factors to their offspring. If we focus on DNA in the nucleus, it would appear that he was right. In each cell, nuclear DNA is bundled into two sets of chromosomes—one from Mom and one from Dad. But mitochondria, the organelles that generate the cell’s energy supply, have their own DNA that comes only from Mom. That means that your mitochondrial DNA is likely the same as your mother’s, grandmother’s, great grandmother’s and so on. Mitochondrial DNA carries far fewer genes than DNA in the nucleus does, but changes in mitochondrial DNA sequence can have a major impact on health. Scientists are studying how variations in mitochondrial genes may lead to disorders of the brain, eye, and skeletal and cardiac muscles.
2.The environment may have the potential to trigger molecular changes that pass from generation to generation.
Gregor Mendel’s studies in pea plants helped launch the field of modern genetics. Credit: Stock image.
Mendel used math to predict how factors that control a pea’s appearance would pass from parent plants to offspring. Environmental conditions experienced by the plants didn’t enter into his equations. New research in a tiny worm called C. elegans, which is commonly used in genetic studies, suggests that it may be possible for environmental stress to trigger small RNA molecules that reduce the activity of specific genes. Scientists think that this gene silencing process, known as RNA interference (RNAi), might help the worms adapt to changing conditions. One study revealed that gene silencing triggered by mild heat stress continued in future generations of these worms, even after the initial heat stress was gone.
3.One trait can be controlled by hundreds of genes.
The traits that Mendel studied in peas, such as pod shape, pea shape and pea color, were each associated with a single gene. Though this is true for some traits, we now know that many traits are controlled by tens or even hundreds of genes spread throughout our genomes. Scientists are finding that some conditions, like preeclampsia, diabetes and asthma, likely involve changes in many genes working in concert.
4.Genes can tag along for generations.
Mendel believed that the factors for different traits are passed down independently of each other. He thought that whether a pea plant passes down a gene for yellow peas, for example, should be unrelated to whether it also passes down a gene that makes the peas wrinkled. For many genes, this rule holds true. However, some genes are close enough together on the same chromosome that they frequently do get passed along together. Genes on completely different chromosomes can also get passed down in groups if they work together in some way to increase an individual’s chances of surviving.
Bypassing Reproductive Cells
Epigenetic marks can pass from parent to offspring in a way that completely bypasses egg or sperm, thus avoiding the epigenetic purging that happens during early development.
Most of us were taught that our traits are hard-coded in the DNA that passes from parent to offspring. Emerging information about epigenetics may lead us to a new understanding of just what inheritance is.
Nurturing behavior in rats
Rat pups who receive high or low nurturing from their mothers develop epigenetic differences that affect their response to stress later in life. When the female pups become mothers themselves, the ones that received high quality care become high nurturing mothers. And the ones that received low quality care become low nurturing mothers. The nurturing behavior itself transmits epigenetic information onto the pups’ DNA, without passing through egg or sperm.
Gestational diabetes
Mammals can experience a hormone-triggered type of diabetes during pregnancy, known as gestational diabetes. When the mother has gestational diabetes, the developing fetus is exposed to high levels of the sugar glucose. High glucose levels trigger epigenetic changes in the daughter’s DNA, increasing the likelihood that she will develop gestational diabetes herself.
Imprinting
In mammals, about 1% of genes escape epigenetic reprogramming through a process called Imprinting.
For most genes, we inherit two working copies — one from mom and one from dad. But with imprinted genes, we inherit only one working copy. Depending on the gene, either the copy from mom or the copy from dad is epigenetically silenced. Silencing usually happens through the addition of methyl groups during egg or sperm formation.
The epigenetic tags on imprinted genes usually stay put for the life of the organism. But they are reset during egg and sperm formation. Regardless of whether they came from mom or dad, certain genes are always silenced in the egg, and others are always silenced in the sperm.
Starved grandpa breeds stronger future generation
