This happens during sexual reproduction, in living things where genes come in pairs.

And it comes down to how reproductive cells are made.

The two genes in a pair can be the same version or different versions.

Same version

And reproductive cells get a copy of one member from each pair.

It’s just as likely to be either one, like the flip of a coin. And it’s a separate coin flip for each gene.

Different versions

Reproductive cell

The chance that two reproductive cells will get the exact same combination of genes is basically zero.

The shuffling is random, so each reproductive cell gets a different combination of gene versions.

That means no two reproductive cells are alike. And it’s why, no matter how many kids two parents have together, each one will be genetically unique.

Now imagine this times 20,000. That’s about how many gene pairs there are in a person.

You can think of recombination as taking all the possible gene versions from two parents and randomly shuffling them into offspring.

And which of these two things an organism does depends on whether it reproduces alone or with a partner.

That's why, aside from identical siblings, every person is genetically unique.

Recombination

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There are two ways this happens: through recombination and mutation.

Mutation

How boring would it be if everyone were genetically identical clones? Thankfully, that’s not likely to happen.

When people reproduce, there are mechanisms built in for making genetic variation. Genetic variation leads to trait variation.

Having genetic variation is so important to a species’ survival that all living things have some way of adding variation when they reproduce.

For living things that reproduce asexually, mutation is the main way they have for making new gene variations.

Now two have the difference.

Trait difference

When this happens, it also makes a new gene version that can be passed on to the next generation.

Mutation acting over many generations can produce multiple versions of a gene.

Reproductive cell

Take this creature—it’s just one cell with some genes inside.

But some of the time, as a copy is being made, a mutation happens. This can be as simple as one DNA building block getting mis-copied, like a typo.

It can just straight-up copy its DNA…

When the critters reproduce again, any differences get copied too.

Each time a living thing copies its genes, there’s a chance of mutations happening that cause new gene differences.

Mutation is universal. It’s a source of variation for all living things, no matter how they reproduce.

…and divide in two to make genetically identical clones.

Even a small change can make a whole new version of a gene. And some of these will cause trait differences.

And one critter ends up with a gene difference.

That’s why, even in living things that reproduce asexually, you can end up with genetic diversity—and trait variations—across a group.

Mutation can happen any time a gene is copied—including during sexual reproduction as reproductive cells are made.

Mutation is how different versions of genes come about in the first place.

Different gene versions

If those critters reproduce, their gene differences get passed down.