Kinship relationships (and DNA)

by Paolo Sirtoli (paolo.sirtoli@gmail.com) - rev 1.0 of july 18th 2018

We all have a very clear idea of what basic kinship relations mean: mother, father, uncle, grandfather, great-grandfather, great-great-grandfather.
On collateral kinship, however, there is a bit of confusion.
If the meaning of cousin is clear (it is the son of a parent's brother), the concept of second cousin is less clear, which is usually understood as "cousin's son".
Beyond these degrees of kinship, usually, they all become generically cousins: the famous "cousins of America".

According to Law

The oldest source of information on kinship is not genetics, but the civil code. In fact, since most civil cases involve inheritance and succession, the Law has long codified family relationships, in order to give certainty to the successors' rights.
The direct line (parent-child descent) is distinguished from the collateral one (between siblings). Affinity, on the other hand, is the link between a spouse and the relatives of the other spouse and therefore is not of genetic interest, but legal (in English, in fact, such ties take the suffix -in-law). According to the Law, the degree of kinship between A and B is the number of steps that separates them in the genealogical tree, i.e. the generation jumps.
According to the codes, therefore, cousins A and B are defined as fourth degree relatives in a collateral line. Let's see how it is calculated: from A it goes up the family tree up to the common relative (the grandfather) and then goes back down to B.
A -> father of A -> grandfather of A and B -> father of B -> B, for a total of 4 degrees. It is interesting to note that the Law recognizes the kinship up to the sixth degree. Beyond the sixth grade, according to the law, there is no blood connection.

Degree

Direct kinships

 

Collateral kinships

     

1

Parent - son  

-

     

2

Grandfather -grandchild   Siblings    

3

GGF - GGC   Uncle - nephew    

4

GGGF - GGGC   Great uncle - GGC
   or
cousins
 

5

GGGGF -GGGGC   GGU - son of GGC
   or
first cousins once removed
 

6

GGGGGF -GGGGGC   GGGU - GGGC
   or
first cousins twice removed
   or
second cousins

In the Anglo-Saxon world

It is important to know the kinship terms used in the Anglo-Saxon world, because all the websites that offer DNA analysis and the major genealogical research sites are written in English.
It should be noted that in the case of cousins, significant errors can be made.
In English, in fact, the link between cousins is expressed in general with two numbers: the degree and the number of removals. First of all we calculate the number of steps that divides A and B from their common ancestor. If this number is the same and the ancestor is the grandfather, then we will simply have two "first cousins" and so far it seems to agree with the Italian meaning.
Two "second cousins" descend for the same number of steps from the same great-grandfather. Pay attention to the difference: in Italian we mean cousin of the second degree the son of the first cousin. Two "third cousins" descend for the same number of steps from the same great-grandfather, and so on. However, if the degrees separating A and B from their common ancestor are not equal, the difference is expressed as the number of removals.
For example, if we talk about the relationship between A and B, which is the son of A's cousin, then we will have to say "first cousin once removed". If B is the son of the cousin's son, then for A he will be a "first cousin twice removed". In practice, the nearest symmetrical relationship determines the degree of "cousinship" and, from there, counting the distances in direct line, we could count "how far- removed" one is.
The following scheme can help. The generations are horizontal with alternating white and gray stripes. In vertical are the relationships in direct line.

 

According to Biology

In scientific terms, each generation builds the genetic makeup mixing the DNA of the father and mother in a random way. As a consequence, each heir will share 50% of the genetic heritage with each parent. So with each generation the degree of affinity with each of the branches is halved.
Homozygous twins are a special case, because their genetic makeup is identical.
As we have already said, for biology there is no related kinship, only that between consanguineous. Therefore it may be useful to superimpose on the previous scheme a number that expresses the percentage of genetic patrimony that two individuals A and B share.
It is clear that at every generational leap the number is halved. It should be emphasized that the degree of kinship between siblings is 50, because there have been two independent mixing of their parents' DNA.
The DNA share would be 100 only for homozygous twins, that are perfectly identical.

 

According to DNA

Modern techniques of analysis are able to measure the degree of similarity of two DNA, which is expressed in centimorgans (cM). Without going deep into the meaning of this unit of measurement, let's just say that this number falls rapidly with the distance between kin: the similarity between father and son is about 3500 cM, while among first cousins drops to 874 cM. Moreover, due to the random shuffling of the DNA, a certain variability in the similarity is expected, therefore the admissible values are between a minimum and a maximum, with a more probable average value. The following table shows the expected values for the different degrees of kinship.


Adapted from: the shared cM project (www.dnapainter.com)

In the Sirtoli family we have had two spectacular confirmations of the genealogical tree so far: in 2016 Paolo Sirtoli performed the DNA test with the kit provided by National Geographic. In 2018 also Cynthia Savage of Kansas City (USA) tested itself. The two did not know each other, but immediately the Family Tree DNA website, on which the data of the two analyzes were uploaded, reported a similarity in their DNA, which hypothesized a distant kinship.
Once they were put in touch, Paolo and Cynthia verified their genealogical trees and the reason for their kinship was clear: they shared their ancestor Felice Sirtoli , born 1819, who had a grandson, Giuseppe , who emigrated to the United States in 1905. Since both Paolo and Cynthia are five steps away from Felice, they are "fourth cousins" and for this reason the resemblance of their DNA is expected to measure 0 to 127 cM, with an average of 35 cM. Well, the figure is 59 cM.
There is a further confirmation: also, Valerio Giannetta in 2017 carried out the DNA analysis and this time we knew in advance the degree of kinship with Paolo. In fact, their common ancestor, Bartolomeo Sirtoli born 1694 is 8 generations away from Paolo and 9 from Valerio: they are "seventh cousins once removed".
In this case the DNA resemblance measure is expected to be between 0 and 53 cM, with an average of 13 cM. The value is 8.5 cM, in perfect agreement with the expected.