picture of Jay Glanville  

Genes Sharing in a Family


Email Jay Glanville 


With a large number of identified relatives, it is interesting to speculate how different (or similar) they are to each other or to myself.

Knowing the "degree of relationship" (or "degree of separation") you can calculate an average amount number of genes shared between any two people.
It is only an average, and there will be a variation around the calculated numbers, but obviously the less shared genes, the less similar they will be, to a point where you may be able to name an individual but in practical terms there is NO likelihood of you sharing ANY genes with them (e.g. William the Conquerer, Julius Caesar).

Do not confuse genes, held in our chronosomes, with D.N.A. - as we all inherit Mitochondrial DNA from our mothers, so in the table below individuals will probably share much more DNA than the genes figure predicts (but it won't affect their features)

The number of genes shared only gives an indication of the similarity as different features expressed in an individual require a different number of genes in order to express that feature - for example facial features require more genes than simply having long or short fingers. Don't overlook that many "similarities" will not be visibily expressed, so people may be more similar than they appear (for example both may be colour blind).
On the other hand, recent reports tell of scientists experiments that found chickens could be developed without beaks, by only modifying two genes - which is far fewer than you or I might have imagined !

It is hard to know how many Genes we each have, research is ongoing and I have seen figures between 19,000 and 30,000, but it appears the concensus that research is leaning more towards lower figures, so I have chosen to use the figure of 20,000 genes per person.

Rather than try to define "Degree of Relationship Separation" I will let the table below speak for itself, with the Relation column giving examples (not a complete list) of people who have that Degree of Relationship.

Degree of Relationship

Degree of Separation Shared Genetic Relation Relationship
% No. of Genes
- 100% 20,000 Identical Twins
1 50% 10,000 Parents or Siblings or Children or Non-Identical Twins
2 25% 5,000 Grandparents or Grandchildren or Uncles or Aunts or Nieces or Nephews or Half-Siblings
3 12.5% 2,500 First Cousins or Great-Grandparents or Great-Grandchildren or Great-Uncles or Great-Aunts or Great-Nieces or Great-Nephews
4 6.25% 1,250 First Cousins once removed or GG-Grandparents or GG-Grandchildren or GG-Uncles/Aunts or GG-Nieces/Nephews
5 3.13% 675 Second Cousins or First Cousins twice removed or 3xG-Grandparents/children or 3xG-Uncles/Aunts or 3xG-Nieces/Nephews
6 1.56% 338 Second Cousins once removed or First Cousins thrice removed or 4xG-Grandparents/children or 4xG-Uncles/Aunts or 4xG-Nieces/Nephews
7 0.78% 169 Third Cousins or Second Cousins twice removed, etc or 5xG-Grandparents/children or 5xG-Uncles/Aunts or 5xG-Nieces/Nephews
8 0.39% 84 Third Cousins once removed or Second Cousins thrice removed, etc or 6xG-Grandparents/children or 6xG-Uncles/Aunts or 6xG-Nieces/Nephews
9 0.20% 42 Fourth Cousins or Third Cousins twice removed etc, or 7xG-Grandparents/children, etc
10 0.10% 21 Fourth Cousins once removed or Third Cousins thrice removed etc, or 8xG-Grandparents/children, etc
11 0.05% 11 Fifth Cousins or Fourth Cousins twice removed etc, or 9xG-Grandparents/children, etc
12 0.02% 5 Fifth Cousins once removed or Fourth Cousins thrice removed etc, or 10xG-Grandparents/children, etc
 Shown another way using this chart from the FamilySearch.org website showing expected ranges for any given relative - centimorgans clearly are comparible with "No. of Genes" above, in numerical terms.

Update following my DNA test in 2021

13,000+ proposed cousinships from my Ancestry test but as of Nov 2022, 143 confirmed by paper-based records from common ancestors as far back as 1713.
The cousinships identified are out to 5th cousins once removed and a 6th cousin with the number of centimorgans shared down to 8.
The table above is discussing Averages, we don't inherit exactly half; sometimes we might inherit all the DNA that is common (I have seen that where both parent and child have taken an Ancestry test), sometimes none, but on average one half.
I think the Ancestry DNA test is confirming a 6th cousinship is about the limit of what DNA can show, and in my own case that means common ancestors born before 1700 are unlikely to be confirmed through DNA testing.


We are unlikely to be able to find photographs of our 8 x Great Grandparents (although perhaps an oil painting may exist). Certainly we are not going to find pictures of our 8 x Great Grandchildren (at least for a while !)
So we are unable to make comparisons of appearances.

However, many genealogists will have made contact with Cousins with a large Degree of Relationship - take Third Cousins for example.
I have a large number of those identified, and quite a few photos to compare, but now we know we can only expect 169 feature-forming genes in common (amongst 20,000), I can understand why they are not as good looking as myself !