Often times we enter discussions regarding genealogy or relatives and we hear such terms as 1st cousin, 2nd cousin, 3rd cousin and beyond. Further complicating the descriptive relationship we hear “once removed”, “twice removed” and so on. We hear these descriptions often and, perhaps, some of us use them not knowing fully what they really mean. 

An excellent explanation of this subject was found on the Internet entitled “Cousins and Cousinhood” written by Frank Arduini. Rather than attempting to write an explanation of these terms and “re-inventing the wheel”, the writer has decided to use Mr. Arduini’s approach.

The following is a chart which illustrates the key points.

Start with the yellow rectangle, it represents you. All the other rectangles represent a blood relative of yours, and within each rectangle the nature of that blood relationship is described.

The first number is your traditional degree of kinship with the person in that block.  This is the way it is legally defined how closely related we are and it is the degree of kinship recognized by both civil and religious authorities. It is calculated by counting the number of steps it takes to move on the chart from that person to you. It’s one step from you to your parents, so you are kin of the 1st degree. 

This way of measuring kinship was developed long before anybody ever heard of Mendelian genetics, so actually it’s not quite accurate in a couple of areas, but not enough to matter to most of us. For our purposes, to simplify the procedure, let’s say that the smaller the number the more closely related you are.

Next, the block names the relationship between that person and you. These will be further explained later.

The last number, shown as a percentage, indicates the actual genetic degree of relationship. This indicates the total percentage of your genes you share with that relative.  For each of your parents you share 50% of their genes. The same holds true with your siblings.

Notice that this is different from your traditional degree of kinship. According to the traditional measure you are 1st degree kin to your parents and children, but only 2nd degree kin to your siblings. In actuality, however, you are equally related to all of them, sharing 50% of your genes with each.

Direct and Collateral Relationships:

We are related to our other family members in one of three ways. We are either direct relatives, collateral relatives or both.

Direct relatives measure lines of direct descent. Anyone in that line will either be a direct ancestor, or a direct descendant of you. In the chart, all these individuals appear as blue rectangles.

Collateral relatives are all others to include siblings, aunts & uncles, nieces & nephews, and all cousins. Collateral relatives share a common ancestor, but are not directly descended from, or ancestral to, each other.

These categories are not mutually exclusive. For example, assume that your great- grandparents were 2nd cousins once removed from each other. That means that all their descendants are related to each other in more than one way. Some of those relationships are direct, and some are not. In this example, you would be your mother’s son/daughter, her 4th cousin, her 5th cousin, and her 4th cousin once removed. Confusing?

“Removed” relationships:

In the chart there is one row of rectangles colored green. This row represents all the individuals on the chart who are members of your generation. Up one row is your parents’ generation and down one row is your children’s. 

Full siblings and cousins are all members of the same generation! In other words, they’ve descended the same number of steps from the shared ancestor. Siblings share a set of parents. 1st cousins share a set of grandparents. 2nd cousins share a set of great-grandparents. And so it goes....

So, if you and your 1st cousin share a set of grandparents, then how are you related to your 1st cousin’s children? They are not in the same generation as you. They are one generation further away from those common ancestors (your grandparents, their great-grandparents). They are one generation removed from your generation.

Now your 1st cousin’s children and your children are in the same generation again, but the shared ancestors are their great-grandparents. That means that they are full 2nd cousins. And your 1st cousin’s grandchildren will be 3rd cousins to your grandchildren, 2nd cousins once removed to your children, and 1st cousins twice removed from you.

As you count the generations from the shared ancestors along each branch, the shortest branch controls the “cousin number.” If the shortest branch only has two generations between the ancestors and the relative in question, then that’s the baseline, 1st cousins. All the generations “removed” are measured from there.

Another way of thinking of it is to move down each generation until you get to the last generation where both branches are the same length. If that is the 1st cousin generation, then all the remaining links along the longer branch will be 1st cousins, as many times removed as they are farther along the branch.

Great Uncles and Grand Aunts:

One of the most common mistakes made has to do with the official name for the relationship between us and the siblings of our grandparents. We tend to call them great aunts and uncles. When we do that we are WRONG!!!

Officially, they are our grand (not great) aunts and uncles. And all the siblings of our great-grandparents are our great grand-aunts and our great grand-uncles.

Halves and Doubles:

One final interesting aside is how we are related to our half siblings, half 1st cousins, etc. We are half as related to half relatives as to our full relatives.

If we share 50% of our genes with our siblings, then we share 25% with our half siblings. Likewise, if we share 12.5% of our genes with our 1st cousins, then we share 6.25% with our half 1st cousins.

Where this gets really complicated is with doubles. For example, Joseph “A” has a sister Mary “A”. Likewise, George “B” has a sister Jane “B”. Joseph “A” marries Jane “B” and George “B” marries Mary “A”. The offspring of each of these marriages are double 1st cousins. They share both sets of grandparents, paternal and maternal. This also means they share twice as many genes as ordinary 1st cousins, fully 25%. Going a step further, if the offspring of each marriage, the double first cousins married, their children
would also be the parents’ 1st cousin once removed. The children of this union, in addition to being siblings, would also be 2nd Cousin to each other.

A final special case is, of course, identical twins. From the genetic perspective, identical twins are really the same person. They share 100% of their genes. Now the children of an identical male twin are as closely related to their uncle as they are to their father. And 1st cousins descended through a pair of identical twins are double 1st cousins, just as if their grandparents had been two pairs of siblings. When two sets of identical twins marry, the 1st cousins are actually quadruple 1st cousins. This makes them, genetically speaking, siblings.

Degree of Relationships: 

The “degree” of a relationship is a legal term. It refers to the number of “steps” between two individuals who are blood relatives. The degree in Civil law represents the total number of steps through the bloodline that separate two individuals. For example, there are two steps from you to your grandparent and then two steps back down to your first cousin, so the degree is four. 

The degree in Canon law measures the maximum number of steps from the nearest common ancestor. Your grandparent is the nearest common ancestor between you and your first cousin, so in this case the degree would be two. Canon law is used in most of the United States. 

Perhaps now we can begin to feel a little more confident when we speak of 3rd cousins six times removed. At least now we have the data to back up our statements.