When a baby is born, three cliché things are done immediately: announcing its gender to the world; counting the kiddo’s fingers and toes; and, inevitably, deciding which parts of its red, angry alien body look like mom and which look like dad.

That last conversation actually never really stops, with quips like “your eyes are just like your mother’s!” and “your nose is just like your father’s!” plaguing every family reunion. Well, next time you want to distract from this family pasttime, drop some science on your relatives and let them know that, genetically speaking, we’re all a lot more like our fathers.

Womb Wars

As anyone who took Biology 101 remembers, we’re all composites of our parents. Mom gives us 50 percent of our DNA and our dad fills in the other half. But only the students who were really paying attention are likely to recall that not all genes are expressed equally. In many mammals, the scales seem to be tipped toward fathers, whose genes often win the war underway in the womb.

This is due in part to the perplexing puzzle known as epigenetics. Basically, epigenetics influence the way your DNA is actually expressed. This can alter your dad’s sperm, which in turn may affect you. It can also affect the way the genes you have are read — and the proteins they produce — across your lifetime.

Take, for example, a 2015 study in Nature Genetics that showed the expression of thousands of different genes in mice varied depending on whether they came from a mom or a dad. While each parent technically contributed half of an offspring’s genome, approximately 60 percent of the dad’s genes were more expressive than the mom’s.

These epigenetic factors can play a role in numerous parts of your life, but they aren’t just about quirks like eye color or whether or not you can roll your tongue. Researchers think differential expression can also change your mental and physical wellbeing. If mom has a predisposition toward a given disease, you may still inherit it. But if your dad passes on genes that pass on an illness or a mutation of some kind, you may be more likely to be sick yourself, simply because his genes are more likely to be expressed.

See also: Men With Poor Diets Produce Unhealthy Children

Baby Daddies

While epigenetic mechanisms are clearly at play, why a father’s genes are more expressive remains unclear. Still, researchers are making headway, and many think it may start with a war all the way back in the womb.

“The fetus can be viewed as a parasite in some ways,” Edward Chuong, a postdoctoral researcher in biology at the University of Utah, tells Inverse. While mom recognizes half of a baby’s genes — those that are similar to her own — the other half, from the father, are alien. That’s why women who are pregnant are immunocompromised and at risk of serious flus and other illnesses. In order to stop themselves from destroying the tiny fetal invader, their bodies have to impair their own healthy immune system responses.

This parasitism is only exacerbated by the fact that dad genes tend to be really aggressive. Evolutionarily, a dad wants his babies to survive and thrive. That way, they’ll continue to pass on his genes, making him evolutionarily fit. But that means that his genes want to pull resources away from the mother, who is — at least in an evolutionary context — only important to a guy insofar as she helps his baby live.

But even that empirically grounded explanation has its limits, Chuong notes. One classic example of this genetic conflict is seen in the fight for expression of insulin-like growth factor 1, or IGF protein. This protein, which promotes growth, is strongly expressed by paternal genes. Maternal genes, meanwhile, express something called IGF2R, which actively suppresses the father’s IGF protein production and, therefore, the baby’s growth.

“In mice, if you knock out the maternal genes, the babies become giant,” Chuong says. They try to outgrow their siblings and extract more and more resources from their mother. Conversely, Chuong says, if you knock out a father’s genes, the babies become small — too small, actually.

Turns out, the mother’s growth-suppressing genes weren’t always there. They only developed in response to the dad’s growth-promoting genes. “It’s like a tug of war,” he says. “These almost cancel each other out.” Without the father’s growth-promoting genes to battle against, a mother’s inhibiting genes can actually hurt the babies. In natural environments, where both the mother and father are contributing the right stuff, offspring come out at optimal sizes.

In the past, other scientists have suggested that dad’s genes are more robust because men need their children to look like them in order to believe they’re really the baby’s father. That makes intuitive evolutionary sense, given that men can’t be certain about their children’s parentage the way women are. But the evidence is actually all over the place, with just as many studies suggesting babies look more like their moms.

While the mysteries of the genome — and the epigenome — have yet to be unraveled, one thing’s clear: Dad’s are pretty important.