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Degrees of Relation and Number of Genes Shared

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.
 
Degree
Genetic relation
Relation
%
No Genes
 
-
100%
30,000
Identical twins
 
1st
50%
15,000
Parents, 
siblings, 
children,
fraternal twins
 
2nd
25%
7,500
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)
 
3rd
12.5%
3,750
First cousins, 
great grandparents, 
great grandchildren, 
great aunts, 
great uncles, 
grandnieces, 
grandnephews,
half-aunts, 
half-uncles, 
half-nephews, 
half-neices
 
4th
6.25%
1,875
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, 
 
5th
3.13%
938
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
 
6th
1.56%
469
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
 
7th
0.78%
235
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.

4. Mitochondrial DNA is a short strand of DNA than is separate to that found on the chromosomes. It is found in tiny bodies inside our cells, the mitochondria, which are responsible for breaking down sugars. We inherit all of our mitochondrial DNA from our mothers and none from our fathers.

5. By "population", I mean breeding population, that is to say the people that your ancestors will have bred with. Prior to the 19th century, there was rather little movement of peoples and gene transfer between those living in, say, rural Yorkshire and rural Devon would have been rather slow. At an even greater extreme, there will have been almost no gene transfer between South Sea Islanders and the Ituri peoples of the Congo basin.

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