What is a haplogroup?
A haplogroup is a set of mutations in the genome located on uniparental chromosomes, which are inherited only from a single parent to the offspring: mitochondrial chromosome and Y chromosome.
Genetic variability occurs mainly for 2 reasons: recombination and mutation. In the case of uniparental DNA (mitochondrial or Y chromosome), it is almost always caused by “de novo” mutations, i.e., mutations that occur for no apparent or known reason. Therefore, haplogroups are quite stable and become a good way to analyze the historical lineage of individuals.
The mitochondrial chromosome defines maternal inheritance, that is, it is inherited from mothers to their sons and daughters. All people, regardless of sex, have a mitochondrial chromosome, since we inherit it from our mother, but only biological women can transmit it to their offspring, so that, although men and women have it, it traces exclusively a maternal lineage.
On the other hand, the Y chromosome defines a paternal inheritance, and is inherited from fathers to sons. It is important to note that it is inherited from fathers to sons, not sons and daughters, so it is present only in biological males. Therefore, the Y chromosome traces the paternal lineage and is exclusive to the male sex.
As we have said, these mutations are perfectly defined throughout history and are very well maintained between generations, so it is very likely that people sharing the same haplogroup have a common relative, which may be from hundreds or thousands of years ago.
Since they are so well defined, it has been possible to determine the relationship that exists between all of them, being able to describe, in the form of a phylogenetic tree*, which were the first to originate and into which ones they diversified. In this way, they allow us to locate quite precisely when this haplogroup originated, based on a multitude of scientific studies from prestigious universities.
Therefore, we can determine at what point in history a very distant relative, common to all those who share a haplogroup, is most likely to be located.
Logically, there are haplogroups that are more frequent in geographical regions, which has made it possible to study the history of human populations, including their migrations and genetic mixtures from a matrilineal and patrilineal perspective.
* A phylogenetic tree, by definition, is a graphic representation of the evolutionary relationships between different species, as if it were a family tree. The branches of the tree (which represent the connections between species and, therefore, their ancestors) diversify from what we call a common ancestor to all the species included from that point. Each time a new species appears, the subdivision of an existing one takes place. If we were to consider all the species that have existed at some point in history, we could say that they all descend from a common ancestral species, which has diversified over time into the current ones. This concept can be extrapolated to other biological levels, whether populations, genes or even haplogroups, as in this case.