To What Extent Can Trauma Be Passed on Through Generations?

To what extent can trauma be passed on through generations?

How can a person have their mother’s laugh, or their father’s intuition, or a sibling’s personality? The human body is made up of cells that contain DNA which contains the genes that get passed on from both parents to offspring. It is long-known from the study of genetics that physical traits such as height, hair color, and eye color all get passed on through genes, sometimes for up to five generations. However, can things like emotional or mental traits such as feelings and emotions be passed on as well? The answer to this is not as simple as the idea of genetic heredity that we already know. Many more factors come to play such as the environment and its role to transgenerational genetics, which leads us to question whether nature or nurture is involved. Does a traumatized mother create a traumatized child? Does a psychotic father produce a psychotic child? And is it genetics or the environment which alter or determine these outcomes? Specifically heading to the subject of trauma, there have been debates on whether trauma can be passed on through DNA. This involves the idea that memories and events from past generations are getting passed on which in some ways are similar to the way our physical traits are formed, and in some ways are totally different from the concept of genetics that is well-known. We will be investigating trauma, or Post Traumatic Stress Disorder (PTSD) specifically, and how it can be passed on through generations. Through this, we can see how children of parents who experienced trauma can develop PTSD because of the role of epigenetic and environmental factors and its effect on transgenerational trauma.

 Deoxyribonucleic acid, also known as DNA, is the carrier of genetic information and is present in almost all living organisms. It is found in each of the fifty trillion cells in the human body. Each cell contains about six to eight feet of DNA which is approximately four-hundred thousand times smaller than itself. This is possible because of the coiling of DNA with the help of proteins called histones, where the DNA is able to fit in such a small limited space like a cell’s nucleus

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. A combination of the DNA and histones is called chromatin. Although the process of creating these chromatins help solve the packaging problem of DNA, it also creates issues when it comes to genetic accessibility. Because the functional units or genes in the DNA are tightly wrapped due to coiling, it becomes less functional, and thus the cell is unable to read them $1^{}$

. This is where the idea of epigenetics comes in. While genes are the ones responsible for creating proteins that basically make up our biological characteristics, epigenetics is the study of how those genes are read and how they are turned on or off. Epigenetic markers with “epi” meaning on top or above, and “genetics” meaning genes, basically mean that they are instructions that are physically located on top of the DNA and histones. These epigenetic marks give instructions to the DNA and signal them to either compress or decompress. These can then affect how the genes are read and can help with the functionality of the genes that are in the DNA. These epigenetic marks are biologically essential when it comes to determining what gets passed on from parent to offspring and what genes are turned on or turned off $6^{}$

. For example, lactose-intolerance is a widely known issue that is caused by the small intestine not producing enough of the enzyme lactase which prevents lactose (sugar found from milk) from getting digested $2^{}$

. Lactose-intolerance is also known to have a great chance of getting transferred genetically. While children of parents who are lactose-intolerant will have a greater chance of developing it because of genetic factors, there is also a chance of not developing it. This is because some genes get turned on and some get turned off, which is mainly the role of epigenetics. Different kinds of cells vary because of epigenetic markers being turned on or off, and it is what helps tells us apart from strangers or how similar we are to family members $2^{}$

. The presence of physical traits such as hair and eye color are determined by whether the marker in the gene is turned on or off, whether they are expressed or not. However, can mental and emotional factors also change an epigenetic marker of a gene, and can it get passed on?

Entering into the subject of trauma, it is usually caused by a deeply disturbing or distressing experience. These experiences can then leave a person with a mental health disorder such as PTSD ${13}^{}$

. It is certain that people who have PTSD, have experienced some sort of discomforting challenge in their lives. For example, events such as wars and natural disasters usually leave psychological effects on the people who have experienced it ${12}^{}$

. However, the trauma does not just end there. Trauma can also affect future generations, as children of parents who have developed PTSD are more likely to develop the same effects of their parents’ traumatic experiences, even without actually experiencing it ${10}^{}$

. Because some gene modifications are possible to be passed on, inheriting trauma through DNA is most likely because of a change in an epigenetic marker in the gene that gets passed on from parent to offspring. This proves that trauma does not just end easily. For example, scientists have always studied this idea even in the past. One of the major effects of the Dutch famine which happened during 1944-1945 was starvation and malnutrition of the majority of the population ${14}^{}$

. People were unhealthy, were more susceptible to illnesses, and they showed more signs of depression. After this incident, a significant amount of children that were born showed the signs and effects of malnutrition, despite having not experienced the famine themselves. It turns out that the effects of the famine which the parents experienced have been passed onto their offspring. The hunger and starvation that the children’s parents experienced have somehow been developed in their genes, which they then passed on to their offspring, even up to two generations after, showing signs of transgenerational experiences ${14}^{}$

.

Furthermore, trauma can be inherited biologically because of epigenetic factors. Because genes can get passed on, changes in epigenetic marks can also be transmitted from generation to generation ${16}^{}$

. In addition, molecular switches and information about how genes are turned on and off are also used to describe gene modifications that are passed on throughout generations. A research work led by Brian Dias at the Emory University School of Medicine in Atlanta proved that trauma in mice can be passed on throughout generations because of changes in DNA. He conducted an experiment on the olfactory senses of mice and trained the mice to associate the smell of cherry blossoms with the fear of receiving an electric shock afterward $4^{}$

. After doing this continuously, changes in a part of the gene in the mice happen, and the mice pass it on to their offspring, changing the genetic code of the next generation mice as well. As a result, the children of the parent mice reacted to the smell of cherry blossoms even without experiencing the shock. In fact, this interesting idea that experiences can be passed on is very useful for animals, especially in the wild. It is the way ancestors inform their offspring that a particular environment is a negative environment for them $4^{}$

. This helps them adapt to the environment and know what things are supposed to be good or bad for them. This is also true for humans, and we call it our “instincts”. Furthermore, scientists found out that the mice experiment effects seemed to last only up to three generations ${17}^{}.$

The first generation offspring were very sensitive to the scent of cherry blossom as they could detect the scent at very low levels and they would try to avoid the odor. The second generation offspring also showed the same level of sensitivity. The third generation showed lower, but still significant sensitivity levels. The sensitivity to the smell disappeared in the fourth generation. A possible hypothesis could be that because it is no longer harmful to the other generations, the gene could have been switched off or changed back to its original state ${17}^{}$

. These types of epigenetic mechanisms prove that traumatic experiences can be passed on,  as epigenetic mechanisms have also been documented in humans.

Recently, the term epigenetics has also been used to define the modifications in the genes that are passed on from parent to offspring. Studies show that the idea of trauma being transgenerational has to do with the change of epigenetic factors in a marker of a gene, but studies on it are shown to be very limited ${17, 18}^{}$

. There are many factors to include in the causes of PTSD and epigenetic modifications can be one. Traumatizing events in the past that have affected many people have shown to have effects of later generations. For example, babies who were growing in their mother’s womb during disastrous events such as the 9/11 terrorist attack turned out to be more prone to developing PTSD because of having lower cortisol levels in the brain ${20}^{}$

. Cortisol is a hormone which is in the glucocorticoid class of genes, meaning that it is one of the hormones that helps control stress response in the brain. Lower cortisol levels in the brain are caused by extreme levels of stress, anxiety, and depression. Mothers who experienced these during pregnancy because of events such as 9/11 were able to somehow transfer and pass on the same effects to their growing offspring, which showed evidence of transgenerational trauma ${7, 20}^{}$

. This effect turned out to be the same when it came to another research on the effects of the Holocaust survivors’ experiences to their children. In a research held by Dr. Rachel Yehuda from the Icahn Medical Institute in Mount Sinai, an experimental study was done on 32 Holocaust survivors and 22 of their children to see if trauma from the disastrous World War II event can be passed onto the offspring who have not experienced such traumatic events at all. It was interesting to see why the children of parents who survived the Holocaust differ from normal children and why they have already shown sufferings from anxiety and stress during their early stages of life, without experiencing trauma at all. It was found out later that both the Holocaust survivors and their children had higher methylation levels in a gene related to stress response, which is a gene that was related to increased risk of developing a psychological disorder like PTSD in people who have experienced some traumatic experiences in their childhood ${19}^{}$

. Dr. Yehuda’s study states that it was not a change of the gene, but rather a change in a marker of the gene that causes methylation levels to change ${19}^{}$

. Specifically, their research team focused on the gene called FKBP5 which is a gene that contributes to the risk of humans developing anxiety, depression, as well as PTSD. Yehuda and her team had seen a pattern from drawing blood from the Holocaust survivors and their children. There was an epigenetic change in their genes. It was not a change to the FKBP5 gene that caused these changes, but rather it was a change in an epigenetic marker that sits on top of that gene ${19}^{}$

.  The survivors of the Holocaust had this change in the marker of their gene and their offsprings who have not experienced any sort of traumatic experience have shown this change in the epigenetic marker as well. This means that the children of the Holocaust survivors have somehow inherited their parents’ traumatic experiences which causes them to be more susceptible to developing anxiety, depression, and PTSD throughout their lives. Although this experiment was concluded to be limited due to the small experiment size, there are ways to further this research. For example, studying more than two generations can help determine whether there was a big change in methylation levels or if it was just higher in that group. Yehuda’s experiment may be considered small and unofficial in some ways, but it suggests that a parent’s life experiences can have an influence on chemical changes in their genes which can then be passed onto their offspring. It may not be the same way as how we get our physical traits from our parents, but more research with more subjects of study can be held in the future to prove a concept that we can identify as memories and experiences that can be biologically inherited by the next generations.

Although the lack of evidence leaves us to question if epigenetic factors play a role in inheriting trauma through DNA, environmental factors also play a role on how trauma can be passed on from one generation to the next $5^{}$

. For example, distressing stories from parents who survived a traumatizing event or parents who experienced traumatizing situations can leave a mark on a child’s perspective and can leave a child more susceptible to having PTSD. This is because parents shape a lot of the environment their kids grow up in which can make a big difference when it comes to their mental health. PTSD is similar to the diagnosis of depression or alcoholism. Having an alcoholic parent makes a child not only more susceptible to being an alcoholic, but it also increases their risk of developing neuroticism, psychiatric stress, and aggressive behavior in their adolescence ${17, 19}^{}$

. However, it makes a difference whether the parent is a recovered alcoholic or not. This means that not only epigenetic factors come into play, but the environment also plays a role in this. It also means that having PTSD does not necessarily mean that your child will have it because there are some ways to prevent it, which will be tackled later in this investigation. Another example pertaining to the topic of PTSD, are the traumatic events such as war. Specifically, for children of Holocaust survivors, having parents who have been through such traumatic experiences and hearing distressing stories about it can be traumatic on its own and therefore increases the risk of the child developing PTSD or trauma. This can be proven because of the symptoms of PTSD. Symptoms such as severe anxiety, negative thoughts, and depression can all leave negative experiences to the person experiencing it, as well as the people who have to live watching someone experiencing it ${17, 19}^{}$

. There is still a risk of this trauma being carried to the next generation even if the research on epigenetic factors of PTSD turns out to be wrong because there is still the environmental side that comes with it.

Additionally, epigenetic marks can also be influenced by the environment as well, which causes environmental factors to play a role in developing trauma in children whose parents and grandparents were exposed to it ${5, 19}^{}$

.This can then help prevent children who are biologically affected by PTSD to overcome the mental disorder. When the environment is stated, it does not just mean the physical environment or the surrounding of the epigenetic marks. Because epigenetic marks are determined before birth or before embryonic development, and before the actual genes are in our DNA, the environment of the outside surroundings during embryonic development plays a role on the epigenetic changes in a gene. For example, the food and vitamins that a mother intakes, or the stress she experiences during pregnancy, all have an environmental factor which causes changes in the child’s DNA or rather, the child’s epigenetic marks. It sends a chemical change within the body which can have an effect on the developing baby ${19}^{}.$

When thought about in a certain way, our genes and our epigenetic marks are already formed during our development in our mother’s womb. However, studies show that epigenetic marks can also accumulate and change after being born because of how our brain develops through experiences as we grow. An experimental study using rats as the subject was done to determine whether this was true. Rats contain a gene called the glucocorticoid receptor or in short the GR gene, which can be expressed and seen in a certain part of a rat’s brain. When it is available to be read, it acts as a help to the rat to cope with stress, which means that the more of the GR gene found in the rat, the better the rat will be with handling and coping with stress. Studies show that this gene can be altered in the early stages of rat brain development, and its relationship with its mother can affect and influence the amount of GR gene that is accumulated. This GR gene has some silencing markers that are attached to it which works effectively to turn off the gene. The rat pups which have received no sort of nurturing from a mother, have in turn developed as an adult which have their GR gene turned off, making those rats grow and develop as angry, anxious, and stressed adult rats. On the other hand, rat mothers who show affection by licking and grooming its pups during their first weeks of life, develop rats that have the GR gene turned on because of the silencing markers that are removed from the gene. This then can help them cope and handle stress throughout their lives ${19, 20}^{}$

. This is a very similar case to humans, as we have this gene as well. Children who grow up with loving and caring parents who may or may not have had PTSD will show better signs of coping and handling with stress in their later lives. Parents who have experienced trauma in their lives and who have children should be able to greatly influence the child’s way of thinking which can influence the child’s chances of getting PTSD. To add to this point, epigenetic marks are also reversible, which is very essential to know because of how PTSD may also be prevented. This is also why scientists of today have developed medications such as serotonergic antidepressants to help cure or decrease symptoms of these mental health disorders. There are also things that people can either do or not do to influence our epigenome. In the topic of PTSD, we want to positively impact the epigenetic factors of children who have parents that have had experienced traumatic experiences and also for children who have not ${5, 12, 13}^{}.$

Likewise, environmental factors can also alter epigenetic regulatory features such as DNA methylation and microRNA expression. For example, factors such as diet, health, smoking, and drinking alcohol, all have effects that can lead to changes in the epigenetic marks of the genes $9^{}$

. This is also why parents who grew up being obese, will more likely develop a child that has higher chances of being overweight. An experiment was conducted with mice to determine if obesity was linked with genetics ${15}^{}$

. Two separate mice sets were experimented on; the first set was the healthy mice fed with a normal and balanced diet, and the second set was genetically modified to have a mutation. This mutation in the gene marker disrupts the hormones for appetite in the mice. It blocks the body’s message of “stop eating” in the brain. This change or mutation is a chemical change in the gene which can also be pertained to as an “epigenetic change” ${19}^{}$

. Studies showed that the mice which were genetically modified to have the mutation consumed around 80 percent more food than the normal mice. The mice grew obese, were bred and their offsprings showed more signs of heart problems, irregular insulin levels, and most developed obesity. For humans, obese parents can pass on their health problems to their offspring as well ${15, 19}^{}$

.This is similar to how memories and past experiences can also be passed on. Because trauma and obesity both have environmental effects on the body, it shares a similar way of inheriting it. This is also why one of the effects of trauma is obesity. There are plenty of studies linking childhood traumatic stress to adult obesity, and it is because both change an epigenetic marker in the gene ${15, 19}^{}$

. Because trauma is stressful, and eating can be a means to relieve stress, as the term “comfort food” is used, people who are depressed or traumatized usually consume food excessively which causes obesity. Transgenerational effects of PTSD such as obesity also help prove how trauma can be affected by the environment.

Furthermore, there is a huge debate on whether it is nature or nurture that is involved with a child’s growth and genes; or is it both which can contribute to the change of the role of PTSD in a child’s life? The truth is, both nature and nurture are involved in the passing of traumatic experiences from one generation to another. In fact, it is not nature versus nurture, but rather, nature and nurture. PTSD can have many negative impacts on anybody’s life, but it does not always have to be terrifying. Having an increased risk factor because of nature does not necessarily mean that the child will have PTSD, but it is still better to keep guard and be prepared, which is why nurture is always important. There are still some ways to prevent PTSD through nurturing. For example, promoting positive parent to child interactions and teaching children some coping skills towards mental issues such as depression and anxiety and other psychological diagnoses including PTSD, can prevent children from developing them. It can also lower the overall risk. Trauma can be life-changing at times, but it does not always have to be a negative experience. People who have suffered PTSD can either ignore it or learn from it. People who learned to cope after traumatic experiences showed more signs of positive growth and development in life compared to people who have not. Things like keeping feelings bottled up and refusing to handle the situations in life will always have an impact on mental disorders, whether it is PTSD or not ${5, 11}^{}.$

Parents who know and have learned to cope with their painful past can prevent their children from experiencing it, even if they do inherit it. There are still some ways to prevent children from developing PTSD such as giving more care, getting medication, asking for professional help, which can then help decrease the effects of PTSD.

The limited evidence that trauma can be genetically transferred from one generation to another is not enough to prove that it is true ${18}^{}$

. However, because of the role of the environmental factors, trauma has a great chance of being transgenerational. It is true that more research is needed to prove that trauma can be transferred from parent to offspring. Researchers also need to work on more than just two generations in order to determine whether high methylation levels were an after-effect of the traumatizing event, or if it was just a product of the past. To display the idea of trauma being inherited biologically does not limit the fact that trauma can be passed on from one generation to the next because of the environmental factors that come with it. Trauma can get passed on whether epigenetics is a factor or not. Environmental factors are more important when it comes to transgenerational trauma. Just the same way doctors can warn us about health risks, knowing that there is a chance of developing mental disorders like PTSD from past generations can also help warn and prevent us from actually developing it. By doing more research on these mental health areas, we can then understand where risk factors come from and learn how to prevent them.

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