We look more like our parents because we share more DNA with them than we do with people who are not in our immediate family. That’s the basic answer, where biological inheritance is concerned.
But we also share, typically, the environments we were raised in with our parents, and often our grandparents. And we do inherit non-biological characteristics from those environments as well. This very subtly introduces a tricky concept in biology called heritability. Heritability is a measure of how much of the difference between people at a population level can be attributed to genetics rather than environment. And it only works at a population level – there isn’t a measure which separates nature, as in DNA, and nurture, as in environment, in individuals.
"You’re much more likely to have blue eyes if both your parents have blue eyes, but it’s perfectly possible for blue-eyed parents to produce brown-eyed kids."
Heritability has been largely based on the study of twins, where you take twins, who are genetically identical, which takes nature out of the equation, and then measure how different they are on specific characteristics – and that identifies how much effect their environment has had on them. We can now do the same thing by sweeping entire genomes for entire groups of people who have a similar characteristic, like a disease or a particular trait.
As is so often the case in genetics, it’s difficult to predict anything about individuals, and much easier about populations. Even things which are thought to be really well understood, like eye colour and hair colour and skin tone and height, are difficult to predict in single children – there are so many factors and variables and genes involved. Broadly, children are more like their parents than they are like random strangers, but specifically, it’s very hard to gauge. You’re much more likely to have blue eyes if both your parents have blue eyes, but it’s perfectly possible for blue-eyed parents to produce brown-eyed kids.
"The most common misapprehension of genetics is that it’s deterministic. It’s not: genetics is probabilistic."
Take something like ginger hair, which is inherited in a very linear, predictable way. It’s still a probability. You can have two brown-haired parents who both carry one copy of the ginger gene. That means that each child they have has a 25% chance of having red hair. But those are independent events. They could have many children, none of whom have red hair, or they could have four children, all of whom do – because each one is a probability. The most common misapprehension of genetics is that it’s deterministic. It’s not: genetics is probabilistic.
Another way to think about it is to ask: why don’t brothers look exactly the same, when they have exactly the same genetic relationship to their parents? The answer is that you get a different mix of 50% of the DNA each time. Both brothers have 50% of their DNA from their mother, and 50% from their father. But unless you’re identical twins, it’s a different 50%. If you take ace to king in diamonds, and you mix it with ace to king in hearts to get one complete set, you’ll still have ace to king, but it’ll be a mixture of hearts and diamonds. If you do it again, you’ll still have ace to king, but a different mix of hearts and diamonds. So overall, it’s a different suit, but with the same numbers. That analogy was crap, wasn’t it?
Adam Rutherford is the author of Creation: The Origin Of Life/The Future Of Life and A Brief History of Everyone Who Ever Lived: The Stories in Our Genes.