Estimates of how many human genes exist range from 25,000 to 100,000. As the sequencing of the human genome nears completion, the consensus is closing in on the suprisingly low figure of around 30,000. The following table uses this number to estimate how many of an individual's genes are shared by their relatives.
Of course, any individual's mother and father share over 99% of their autosomal[2] DNA (the DNA not carried on the X and Y[3] chromosomes[1]). This means that the individual shares much more of each parent's DNA than the 50% indicated by the table.
Thus the table shows
us that 50% of our DNA is directly received from each parent and not that the
genes inherited from one parent are entirely different to those inherited from the other.
What the table also shows us is that, beyond 7-degrees of relation, the number
of genes shared in this manner is rather few. By extension, an individual's
sixteenth-century, 13 times great grandfather has less than a half chance
of providing a single autosomal gene to that individual, assuming that there is no
other relationship between the two.
In terms of genes, there
isn't much significance in taking your family tree back to Norman times
for, at that distance, our genetic heritage will long ago have merged into
the population's gene pool[5]. In fact, ignoring the DNA that all human's share,
it's possible to be genetically unrelated to a direct but distant ancestor even when hanky-panky can be
ruled out!
There are, however, three genetic lineages which can be traced back much further.
The first is your pure maternal line,
i.e. your mother's, mother's, mother's etc. lineage. This is because you inherit
a small fragment of DNA, called mitochondrial DNA[4], solely from your mother.
The second is a man's pure paternal line. This is because a man inherits
his Y-chromosome[3] solely from his father. This does not apply to
women as they do not have a Y-chromosome.
The third are specific genetic markers that can be pinned down to specific populations. An example
of this is the gene associated with cystic fibrosis, the origin of which can almost certainly be traced
back to European populations. Most such genetic markers will not be associated with disease although,
to date, these have been the best studied.
The amount of DNA involved with first two of these lineages is rather little compared to the rest
of our DNA. However, because of its unique status, it is often analysed for genealogical purposes.
The third group of lineages represent a much larger, more diverse, number of genes, but, as genetic
typing advances, I believe it will play a much larger role in genealogy.
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Identical twins |
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Parents,
siblings, children, fraternal twins |
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Grandparents,
grandchildren, aunts, uncles, nieces, nephews, half-siblings, double-cousins (children of 2 siblings x 2 other siblings), identical twin cousins (children of identical twins) |
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First
cousins,
great grandparents, great grandchildren, great aunts, great uncles, grandnieces, grandnephews, half-aunts, half-uncles, half-nephews, half-neices |
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Great
great grandparents,
great great grandchildren, great great aunts, great great uncles, great grandnieces, great grandnephews, first cousin once removed, great half-aunts, great half-uncles, half-grandnephews, half-grandneices, |
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Great
great great grandparents,
great great great grandchildren, second cousins, first cousin twice removed, great great great uncles, great great great aunts, great great grandnieces, great great grandnephews great great half-aunts, great great half-uncles, great half-grandnephews, great half-grandneices |
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Great
great great great grandparents,
great great great great grandchildren, second cousins once removed, first cousin thrice removed, great great great great uncles, great great great great aunts, great great great grandnieces, great great great grandnephews great great great half-aunts, great great great half-uncles, great great half-grandnephews, great great half-grandneices |
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Great
great great great great grandparents,
great great great great great grandchildren, third cousins, second cousins twice removed, first cousin four times removed, great great great great great uncles, great great great great great aunts, great great great great grandnieces, great great great great grandnephews great great great great half-aunts, great great great great half-uncles, great great great half-grandnephews, great great great half-grandneices |
Notes:
1. Chromosomes are the large sections of DNA found in the middle
(the nucleus) of all of our cells. Humans have 23 pairs of chromosomes which contains
the vast majority of our DNA - the only exception being the mitochondrial DNA (see 4 below).
2. Autosomes are all the chromosomes excluding the X and Y chromosomes. When
egg and sperm cells are manufactured, the genes on the complimentary pairs of autosomes are
shuffled together to make a complete but unpaired set which become an egg or sperm together with
an X or Y chromsome as described below.
3. The X and Y chromosomes are chromosomes which, in reproductive terms, act
differently to the autosomes mentioned above. Women have a pair of X chromosomes which behave
similarly to the autosomes in that their genes are shuffled during egg production.
Men on, the other hand, have only a single X chromosome paired with the much smaller
Y chromosome. The Y chromosome contains only a handful of genes, most of which are not
shuffled during sperm production. One of the genes on the Y chromosome determines that the
bearer will develop as a male. Men cannot shuffle their X chromosome either, as they only have
a single copy. Mothers always pass an X chromosome to their children while
a man passes an X chromosome to his daughters and a Y to his sons.
Comments, corrections, queries and additions gratefully received at:
l a e t o l i @ u k o n l i n e . c o . u k