Page 9 - O Mahony Journal 2025

systems, their definitions are different. That is, the names of the mitochondrial DNA
(mtDNA) haplogroups do not correspond with the same names of the Y-chromosome
DNA (Y-DNA) haplogroups.

• Clade
A clade is a group of related individuals.

• Sister Clade
A sister clade is one of two haplogroups or subclades that are at the same level on
a phylogenetic tree. For Y-chromosome research, this is sometimes a brother clade.
For example, on the maternal tree, H6a and H6b are sister clades.

• Subclade
A subclade is a subgrouping in the haplogroups of the human genetic trees. This
may be either the Y-chromosome tree or the mitochondrial tree. Subclades are more
specific to a location or population group than the major branches (haplogroups).

Once he had defined some basic terminology, John deftly took us to a brief exploration of the three DNA
tests used by genealogists: Autosomal, Mitochondrial, and Y-DNA.

There are three sources of information in a DNA sample. Y-chromosomal DNA (Y-DNA)
is present only in samples from males and gives information on patrilineal descent.
Mitochondrial DNA (mtDNA), present in both males and females, gives information
on matrilineal descent. Finally, autosomal DNA (atDNA) gives information on both
matrilineal and patrilineal descent.

The signal of shared ancestry seen in autosomal DNA is highest in close relatives but
dilutes quickly so that by 5-7 generations of separation, it is difficult to distinguish exact
relationships other than shared ethnic affinities. Thus, autosomal DNA (atDNA) is best
to help identify ancestors within the most recent 5–7 generations of a family tree.

MtDNA and Y-DNA tests are limited to relationships along a strict female line and a
strict male line, respectively.

In reading through the literature about mtDNA, you will find mention made of
mitochondrial DNA being much more subject to mutation than Y-chromosome DNA.
This description may be somewhat misleading. Y-chromosome DNA is usually very
slow to mutate (change); each change may take thousands of years. mtDNA may be
faster to change than Y-chromosome DNA but the interval between mutations can still
be multiple centuries.

MtDNA and Y-DNA tests are utilized to identify archeological cultures and migration
paths of a person's ancestors along a strict mother's line or a strict father's line. Based
on MtDNA and Y-DNA, a person's haplogroup(s) can be identified. (A haplogroup is
DNA or Chromosomal segments derived from a group of people who share a common
genetic ancestor). The mtDNA test can be taken by both males and females, because
everyone inherits their mtDNA from their mother, as the mitochondrial DNA is found
within the egg cell. However, a Y-DNA test can only be taken by a male, as only males
have a Y-chromosome.

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