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Haplogroups, Haplotypes and genealogy, oh my

I received some criticism the other day over my own criticism of the use of haplogroups in genealogy — the finding and tracing of relatives. My language was imprecise so I want to make a correction and explore the issue in a bit more detail.

One of the most basic facts of inheritance is that while most of your DNA is a mishmash of your parents (and all their ancestors before them) two pieces of DNA are passed down almost unchanged. One is the mitochondrial DNA, which is passed down from the mother to all her children. The other is the Y chromosome, which is passed down directly from father to son. Girls don’t get one. Most of the mother’s X chromosome is passed down unchanged to her sons (but not her daughters) but of course they can’t pass it unchanged to anybody.

This allow us to track the ancestry of two lines. The maternal line tracks your mother, her mother, her mother, her mother and so on. The paternal line tracks your father, his father and so on. The paternal line should, in theory, match the surname, but for various reasons it sometimes doesn’t. Females don’t have a Y, but they can often find out what Y their father had if they can sequence a sample from him, his sons, his brothers and other male relatives who share his surname.

The ability to do this got people very excited. DNA that can be tracked back arbitrarily far in time has become very useful for the study of human migrations and population genetics. The DNA is normally passed down completely but every so often there is a mutation. These mutations, if they don’t kill you, are passed down. The various collections of mutations are formed into a tree, and the branches of the tree are known as haplogroups. For both kinds of DNA, there are around a couple of hundred haplogroups commonly identified. Many DNA testing companies will look at your DNA and tell you your MTDNA haplogroup, and if male, your Y haplogroup.  read more »