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How is DNA used in genealogy?
The sperm and egg cells each contain one of each kind of chromosome, which may
be the one inherited from the mother or that from the father. Since there are
two possibilities for each of the 23 chromosomes, there are 223 different
possibilities for an egg or sperm cell; this is more than 8 million. In
fertilization, one of the 8 million possible sperm cells combines with one of 8
million possible kinds of egg cell, resulting in an enormous number of possible
chromosome combinations in the fertilized egg. Furthermore, in the process of
cell division leading to the formation of sperm and egg cells, the chromosome
pairs within the man and the woman may exchange bits of themselves with their
pair-partners, resulting in even more variety in the chromosomes of the
resulting child. This enormous variety accounts for the essential uniqueness of
each person, and of that person’s DNA. This is why most DNA is useful for
matching a forensic sample with a suspect, or for determining the paternity of a
child. However, this same variability makes most DNA relatively useless in
determining relationships more distant than siblings or first cousins, and thus
relatively useless for genealogical investigations.
There are two exceptions to this situation, however: mitochondrial DNA which is
inherited only from the mother, and Y-chromosome DNA inherited only from the
father.
Mitochondria are tiny structures located outside the nucleus of the cell, which
have their own DNA. At conception, only the nucleus of the sperm cell enters the
egg cell; the remainder, including the sperm cell’s mitochondria, is discarded.
Thus, the only mitochondria inherited by the child are those of the mother. In
principle, a child’s mitochondrial DNA is identical to that of its mother, which
in turn is identical to that of the mother’s mother, and so on back through the
ages. This is not quite true in practice, because of rarely occurring changes,
or mutations, which may occur from one generation to the next.
A similar situation occurs with Y-chromosome DNA. Since a woman has no
Y-chromosome, a boy’s Y-chromosome can only come from his father. Furthermore,
unlike the other chromosome pairs, the Y-chromosome exchanges no genetic
information with its pair-partner the X-chromosome. Thus, but for occasional
mutations, a boy’s Y-chromosome is identical to his father’s, and to his
father’s father’s, and so on. This ancestral line through which the Y-chromosome
is inherited will be referred to as the male line. (The term Y-line has also
been used for this. In both cases, it should be understood that “father” means
the biological father.)
It is the mutations which are key to genealogical investigations. If the
mutation rate is known (or can be closely estimated), this information can be
used to estimate the number of generations back to a common male-line ancestor
for two men whose Y-chromosome DNA is identical or nearly identical (and the
same can be done for two women whose mitochondrial DNA is similarly close).
Since mutations occur randomly (and the “rate” is only an average rate), the
answers to these questions must be stated in terms of probability. Tests of
Y-chromosome DNA can be used to answer questions such as:
What are the chances (what is the probability) that two men have a common
male-line ancestor within a given number of generations?
How many generations back must we go to have at least a 50% (or any other
given percent) chance that these men have a common male-line ancestor within
that number of generations?
Answers to questions such as these cannot prove with certainty that two men
actually have a “recent” common male-line ancestor (within, say, at most 10-20
generations), much less identify that ancestor, but a fairly high probability
may do the following things:
(1) It can add significantly to our confidence in the correctness of documentary
genealogical research which has identified such an ancestor.
(2) It can point to a possibly fruitful direction for future research. For
example, if two men, not known to be related, are shown to have a probable
common ancestor, then searching for the ancestors of the second man may lead to
finding ancestors of the first.
On the other hand, a low probability can suggest that two men are, at best, only
very distantly related, that there is little hope of finding a recent common
male-line ancestor, or that research identifying such an ancestor is incorrect
because at some point the biological ancestor differs from the ancestor of
record. In such cases, further DNA testing may help to locate where the break
occurred, and thus facilitate discovery of the correct ancestor.
These probabilities were investigated by Dr. Bruce Walsh of the Department of
Ecology and Evolutionary Biology, University of Arizona. The mathematical
computations of them were presented in the article “Estimating the time to the
MRCA for the Y-chromosome or mtDNA for a pair of individuals,” Genetics 158:
897-912. Dr. Walsh’s results are presented graphically on-line in his
Time to Most
Recent Common Ancestry Calculator.
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