This is a difficult subject for just about everybody except perhaps avid genealogists. There might not be any discussion here. If not, this will serve as a tutorial.
Let's start with a chart. You are the white box. You understand what siblings, children, parents, grandchildren, grandparents, great grandparents, (first) cousins, and aunt/uncles and probably great aunts/uncles are.
It's all of these other cousin names that are the problem:
First note that adding grand to a parent or great followed by grands to children removes them one more generation each. These are all ancestors or descendants (assorted aunts, uncles and cousins are not).
Next, note that a first cousin is somebody with common grandparents (also, the offspring of an aunt and uncle). A second cousin is somebody with whom you have common great-grandparent, a third cousin is somebody with whom you have common great-great grandparents, and so on.
Next, note that as we name descendants of these various degrees of cousin, they stay second or third cousins, but we add a degree of removal for each next generation, so your second cousin's child is also a second cousin, but once removed, and your second cousin's grandchild is also a second cousin, but twice removed.
On the other hand, when you go up on the chart with any degree of cousin, they don't stay second or third cousins or whatever. They drop a cousin number while adding a degree of removal.
Let's look at second cousins - three boxes to the right of "SELF." Their children stay second cousins, but now once removed, whereas their parents become first cousins once removed.
Likewise with third cousins. Their children are also third cousins, but once removed, and their great grandchildren remain third cousins, but twice removed. However, their parents become second cousins once removed and their grandparents lose another cousin number but add a removal number: first cousins twice removed.
Now back to the thread title: The consequences of this are that two relatives can be called second cousin once removed: the child of a second cousin (cousin number doesn't change when we go down in the chart, just removal number) and the parent of a third cousin (going up in the chart decreases cousin number while increasing the removal number).
And those numbers at the bottom of the graphic - the percentage of genetic similarity - are useful in forensic genealogy. They've got the DNA of a killer, but not his name. Suppose that the perp has 25% of his genome in common with a few 23 and Me or Ancestry.com customers. That degree of relatedness is indicated by orange here. The perp is therefore a person on the registry's grandparent, grandchild, aunt or uncle, or niece or nephew.
You can learn this!
Let's start with a chart. You are the white box. You understand what siblings, children, parents, grandchildren, grandparents, great grandparents, (first) cousins, and aunt/uncles and probably great aunts/uncles are.
It's all of these other cousin names that are the problem:
First note that adding grand to a parent or great followed by grands to children removes them one more generation each. These are all ancestors or descendants (assorted aunts, uncles and cousins are not).
Next, note that a first cousin is somebody with common grandparents (also, the offspring of an aunt and uncle). A second cousin is somebody with whom you have common great-grandparent, a third cousin is somebody with whom you have common great-great grandparents, and so on.
Next, note that as we name descendants of these various degrees of cousin, they stay second or third cousins, but we add a degree of removal for each next generation, so your second cousin's child is also a second cousin, but once removed, and your second cousin's grandchild is also a second cousin, but twice removed.
On the other hand, when you go up on the chart with any degree of cousin, they don't stay second or third cousins or whatever. They drop a cousin number while adding a degree of removal.
Let's look at second cousins - three boxes to the right of "SELF." Their children stay second cousins, but now once removed, whereas their parents become first cousins once removed.
Likewise with third cousins. Their children are also third cousins, but once removed, and their great grandchildren remain third cousins, but twice removed. However, their parents become second cousins once removed and their grandparents lose another cousin number but add a removal number: first cousins twice removed.
Now back to the thread title: The consequences of this are that two relatives can be called second cousin once removed: the child of a second cousin (cousin number doesn't change when we go down in the chart, just removal number) and the parent of a third cousin (going up in the chart decreases cousin number while increasing the removal number).
And those numbers at the bottom of the graphic - the percentage of genetic similarity - are useful in forensic genealogy. They've got the DNA of a killer, but not his name. Suppose that the perp has 25% of his genome in common with a few 23 and Me or Ancestry.com customers. That degree of relatedness is indicated by orange here. The perp is therefore a person on the registry's grandparent, grandchild, aunt or uncle, or niece or nephew.
You can learn this!
