Since Maya was born, people have been constantly opining which one of us she looks most like. Some see in her the spitting image of my family’s characteristics, some see Shanthala in her, some say that she looks like me but her eyes are unlike either of us, Shanthala herself says that Maya’s chin is unlike either of us. Comments pass from her features to her skin color. She’s much fairer than either of you, some have said while others have said that she has Shanthala’s skin tone. And then they’re amazed at her height (she’s in the 95th percentile for her age and sex) and wonder where that characteristic came from.

Shanthala and I joke (with a hint of seriousness) about the roots of Maya’s seeming impatience or her easy, smiling nature. My parents compare her to my sister in terms of how easy she seems to be to care for. Many say that girls are easier to raise than boys. Maya had a lot of eczema till recently. While she never seemed bothered by it, Shanthala and her parents worried that her skin maybe as sensitive as Shanthala’s was and that she may suffer from skin irritations.

All this is boringly normal, this ongoing back-and-forth between who a child looks more like and which characteristic comes from whom. But it appears that the battle rages inside in very fascinating ways.

It is well understood that a child inherits half the genes from the biological mother and half from the biological father (in a world of donor eggs, donor sperms, surrogate mothers and adopted children, the simple terms mother and father have become too narrow to capture the reality). Typically both copies of the genes are active in the child. But in a less than one percentage of them, one of the copies is turned off. In some cases, it is the mother’s copy that is turned off and in some cases, it is the father’s. Wait, you say, isn’t this what I learned in school about dominant and recessive alleles ? About how blue eyes are a dominant gene over brown eyes and so gets expressed ? Isn’t this what you’re talking about ? Sure, this is true only if both copies differ from each other, not if they’re similar. Also, the discussion revolved around whether a particular gene was dominant over the other.

Here is where a newly discovered technique differs. In some small percentage of cases, the same gene sequence will express a different behavior in the offspring depending on which parent’s copy is turned off. For example, if the copy comes from the mother it produces blue eyes and if it comes from the father, it produces brown eyes (the color of the eyes is used for illustration, it is not a real example of this technique). This seems absurd. The same gene sequence should produce the same behavior, independent of the source of the gene. This mechanism of inheritance where the behavior is determined by which copy of the parent the gene comes from is called gene imprinting (or genomic imprinting) i.e. which parent’s imprint is retained in the offspring’s copy. It is now considered a separate process of inheritance compared to the classical Mendelian genetic inheritance.

But wait, gene imprinting gets even more interesting. In an interview with the magazine Edge, the evolutionary geneticist explains:

This is a complicated process because the imprint can be erased and reset. For example, the maternal genes in my body when I pass them on to my children are going to be paternal genes having paternal behavior. If my daughter passes on paternal genes to her children, even though she got the gene as a paternal gene from me it would be a maternal gene to her own offspring. Molecular biologists are particularly interested in understanding the nature of these imprints, and how it is possible to modify DNA in some way that is heritable but can then be reset.

This would be merely fascinating if it didn’t have startling consequences on the offspring. There are children who always smile and laugh, but sadly also have symptoms that are similar to those with severe autism and they mostly never learn to read or write. Some other children almost not nurse as infants forcing them to be tube fed but in a few years time, they develop an insatiable appetite and develop schizophrenia. The first suffer from a genetic disorder called Angelman syndrome and the latter suffer from a genetic disorder called Prader-Willi Syndrome, the first is caused by mutations to the paternal gene imprinting and the latter is caused by mutations to maternal gene imprinting. Many scientists now speculate gene imprinting to be the cause of many problems that plague us humans from asthma and diabetes to cancer. The reason for this is that one of the copies from the parent is bad, the other copy is present to be used. But in case of gene imprinting, the good copy is turned off and only the bad copy retained, making the offspring more susceptible to disorders.

In 1999, David Haig offered an intriguing hypothesis that suggested that in the battle for imprinting the child, each parental genome approached the union with a view that was beneficial to their side. Evolutionarily, for a mother, it is important to spread her efforts amongst all her children, striking a balance between putting all her eggs into a single child and spreading herself too thin by having too many of them. For a father on the other hand, it was more useful to ensure that his child got the most attention (and resources) from the mother compared to other children (biologically, monogamy is a culturally induced trance, and one that is frequently broken). Thus, mother’s side of the gene pool was growth moderation while the father’s side was growth promotion.

Interestingly, gene imprinting is not known in animals other than mammals. Platypus, for example, the earliest mammals, do not have any genes that are imprinted.

In a recent article in the science magazine, Discover, the excellent science writer, Carl Zimmer, explains all this and takes it one step further. He quotes the work of two evolutionary biologists, Bernard Crespi and Christopher Badcock, who suggest that “our minds too are shaped by this battle between the parental genes”. They hypothesize that autism and schizophrenia are the two faces of this evolutionary conflict between the parental genes expressed in the brain. Carl writes:

One of the most striking contrasts between autism and schizophrenia is how they affect the ability to understand others. Autistic people have a difficult time figuring out what other people are feeling. Schizophrenic people, on the other hand, sometimes do too good a job. They may come to believe that a refrigerator is talking to them, for example, or that people are conspiring against them.

Crespi and Badcock propose that these symptoms result from the genetic conflict. Empathetic children can see how frazzled they’re making their mothers and how much attention their siblings need. Maternal genes should therefore boost our abilities to get inside other people’s heads. Paternal genes, on the other hand, may benefit by reducing these distractions from the business of getting more resources from mothers.

Life is such a wondrous miracle. I’ve often times felt puzzled about how people can accept a simple explanation such as god when there are such fascinating explorations and reasons which can explain how this miracle actually happens. Most of the time, all goes well and we end up with a healthy, normal child. Most of the time. On this Thanksgiving Day, I want to offer thanks and express gratitude for the turns of time that have placed Shanthala and I where we are, with who we are and how we are. And of course for the healthy gift of life that almost didn’t happen, Maya.

References:
- Imprinted And More Equal, American Scientist, 2007
- GeneImprint: The main technical website that discusses everything pertaining to gene imprinting
- Alleles & Inheritance: A webpage discussing the difference between normal and imprinting inheritance

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