Theyre like genetic markers for certain populations, but why are there different haplogroups? Do they happen from genetic mutations? And why are there subclades for haplogroups? Do they occur from a similar process of mutation?
Does it say much about the origins of a population?
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Human beings inherit our genetic material from our parents: 23 chromosomes from mom, 23 chromosomes from dad. Our parents have 2 different chromosomes of each kind, and we get one of them at random. But it’s not even that simple, because a process called “crossover” means that each chromosome you inherit probably has a little bit of the other chromosome of that same parent, too.
But wait! Every man has precisely one Y chromosome, which he inherited from his father. His father also had precisely one Y chromosome. All things being equal, then a man should have the exact same Y chromosome as all of his male-line ancestors.
Of course, not all things *are* equal. The process of copying out chromosomes for our children is imperfect, which leads to mutations. This means that a father might have two sons with very slightly different Y chromosomes. Each of those sons will transmit their Y chromosome down, and so all of their sons and grandsons etc. will have Y chromosomes that are slightly different from each other.
Simply speaking, a haplotype refers to a specific variety of Y chromosome. It actually refers to particular versions of several specific regions of the Y chromosome, but thinking about it as a flavor of Y chromosome is close enough. It turns out that all of the men of a similar ancestry tend to have the same haplotype, or one of a small number (like, 3-5). This is for interesting reasons related to random graphs and exponential growth that are beyond the scope of this answer.
Anyway, what you can do with haplotypes is put them in a tree, much like we do with species. Wolves are closely related to coyotes, less closely related to jackals, less closely still related to red foxes, who are themselves close to arctic foxes.
Haplotypes are some arbitrary level of this classification – maybe corresponding to “goats” or “two-hooved mammals” or something.
“Subclade” literally just means subgroup. Sometimes you want to refer to a particular subtype of a particular haplotype: maybe you’re looking at a haplotype of native South Americans, and you’re interested in the difference between those that lived in Peru and those that lived in Bolivia or something.
We can infer some things about population origins because if two populations have similar haplotypes, it means that they were probably the same population not that long ago. So, say, we look at the Malagasy of Madagascar and we see that their haplotypes are a mixture of those from Indonesia and those from Eastern Africa; we then say “the Malagasy probably originated from the mixture of migrants from Eastern Africa and Indonesia”. Ashkenazi Jews have lived in Europe for a couple thousand years, but their haplotypes are still mostly those of the Levant, so we say that the Ashkenazi Jewish population probably originated in the Levant and didn’t marry in many men.
A similar process can be done with mitochondrial DNA, which is inherited exclusively from your mother. That’s also called haplotype now, though it wasn’t at first. That actually sometimes lets us know interesting things like the fact that Ashkenazi Jews didn’t bring in many local men, but **did** marry European women.
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