[ema]

We inherit the genes of our parents not completely mixed but in large chunks. So someone who is first generation mixed race while have large continuous regions of their genome from one race or the other. If two such people have children they will have the same amount of genes from each race as their parents[1] but the continuous regions will be smaller. So from the average length of regions which we can attribute to different founder populations we can estimate how long ago the admixture happened.

[1] This is not completely true as we seldom inherit exactly 25% of our genome from any grandparent, but it is usually pretty close.