It’s all in the family! The biology of inheritance, part 1

Author: Shweta Ramdas

Editors: Molly Kozminsky, Jimmy Brancho, Kevin Boehnke

Harry Potter has his mother’s eyes. From his father, James, he inherits his black hair, his ability to play Quidditch, and a certain predisposition to mischief. We are all unique combinations of our parents, receiving half our DNA from each. In the genetic lottery, our parents’ genes are scrambled and spliced to create a new individual who carries on the family’s long tradition of snoring into one’s sheets. But besides a rickety knee, shortness of stature, and preferred pizza toppings, what else can we blame on our parents?

Replication begets mutation

Mutations, for one. Not only do we inherit some of our parents’ mutations, our DNA contains additional mutations that our parents don’t have, but still pass to us! These are called ‘germline’ mutations because they occur in the ‘germ cells’ of our parents—eggs from our mothers and sperm from our fathers. Each of us is born with 30 such germline mutations on average, but germline mutations accumulate with age, so older parents pass on more to their children. The parental contribution to germline mutations is unequal—the father’s age has more influence than the mother’s. This is because more cell divisions lead to a greater chance of mutations, and since sperm is produced by a constant division of cells starting at puberty, sperm from older men bear a greater mutational load. This is one of the major reasons autism is linked to a higher paternal age. Women, on the other hand, produce their lifetime supply of eggs while they are still in the womb; thus, all eggs have undergone the same number of cell divisions, regardless of the woman’s age.

Epigenetics of life experience

However, it’s not just mutations that are transmitted across generations. Epigenetics (dynamic changes to DNA regulation) ensure that parental behavior and experiences affect offspring: children can be affected by their parents’ smoking, diet and stress. And that’s not all. Eerily, mice have been found to inherit scent associations from their mothers. Mice trained to fear the scent of cherries and almonds passed on this fear to their pups, even if the pups had never encountered the scent before.

One possible reason for this is the inheritance of microRNAs. MicroRNAs are small molecules that turn off certain genes in the cell, and can be transferred from the mother through her egg. These microRNAs could turn off those same genes in the pup, leading to fear, even though the pup didn’t have the painful experience. This particular case of sensory reaction inheritance has led scientists to speculate that habits or preference could be transmitted from parent to child. While the current paradigm suggests that taste or smell preferences run in families because of shared experiences and exposure, this scent association study demonstrates a plausible genetic reason for such shared preferences.

Genetic traces of events in one’s life can also be transmitted across multiple generations, according to research from Sweden. Researchers found that men lived longer and were less likely to have diabetes when their grandfathers experienced famine (as opposed to abundance) during pre-puberty. They saw the same patterns for women, but they depended on their grandmothers’ experiences while in the womb (i.e., when their eggs were forming). Our grandparents’ fasting or feasting at the time their germ cells were being formed can influence our health!

Our parents pass on half their DNA to us, and this together with various mishaps (mutations!) and mechanisms (epigenetic changes!) go on to make us a unique combination of their own selves. But since we share both our genes and our environments with our parents, how do we know if something is passed on to us biologically, or if it’s merely the environment? For instance, could a mother and her son both be diabetic because of a genetic risk factor for diabetes, or is it simply because they both have a sweet tooth? In part 2 of “The biology of inheritance”, we’ll talk about how we measure this. Until then, don’t be surprised if–cursed or not—Harry Potter’s children end up as adventurous and treacle-tart loving as their parents.

About the author

Shweta is a graduate student in Bioinformatics at the University of Michigan. Her research involves computational methods to understand the genetic basis of psychiatric disease. Her undergraduate degree is from the National University of Singapore where she studied computational biology. Outside of research, Shweta enjoys reading, yoga, and figuring out the genetic basis for being a muggle. Follow Shweta on Twitter.

Read all posts by Shweta here.

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Comic – http://www.xkcd.com/634/