We have words to describe the degree of relatedness. How close is a full-bloodied sibling? Two siblings have four grandparents in common. This is the gene pool that they are sharing. So, of the 1024 genes that we might say the grandparents in total have, each sibling could have any one of these genes. Which is just another way of saying they share in all of the genes. So the gene sharing of the siblings is 100%. They do not have the same genes; each sibling is different. But the pool that they draw from is the same. In theory, but it is a remote possibility, two siblings could be identical from the random assignment of the genes.
The only thing closer than normal siblings are identical twins. Not only do they share in the same gene pool, with the possibility of having the same genes, they actually , as an accident of nature, do have the same genes. Cloning yields identical twins by a slightly different process. Twins, or cloning, are, as the mathematicians say, the identity operator. You operate on A and get B, an identical copy.
Excepting the twins and clones, siblings are the closest relationship possible. They share 100% of the available gene pool.
Let us take two half-siblings. They share only two of the four grandparents. Their degree of relatedness is 50%. The gene pool that is common to each one is 50% of the total gene pool at the grandparent level.
Now let us consider first cousins. They share two of the four grandparents that each of them has. So first cousins have the same degree of relatedness that half-siblings do. Their sharing in the gene pool is 50%.
Next, let us take double-first cousins as arose when Michael Käfer married the widow Anna Maria (Blankenbaker) Thomas. Now Michael’s sister Apollonia had married John Nicholas Blankenbaker, the brother of Anna Maria (Blankenbaker) Thomas. So we have a brother and sister marrying a sister and brother, respectively. (It could have been two brothers marrying two sisters also.) All of the children of these two couples share the same four grandparents. That is the gene pool which is available, in principle, to each one of them. In other words they share 100%, the same as siblings. Thus, siblings and double-first cousins are equivalent.
The next step away, after siblings or double-first cousins, is half-siblings and first cousins at 50%.
Here is the homework assignment. Suppose two people, at the fifth generation earlier than they are, have four ancestral pairs in common. Hint: How many ancestral pairs are there at the previous fifth generation? What fraction of these are shared?
(09 Sep 02)
We gratefully acknowledge the work of John Blankenbaker who published over 2,500 Germanna History Notes via the Germanna-L@rootsweb.com email list from 1997 to 2008. We are equally thankful to George Durman (Sgt. George) for hosting the list and republishing the notes via rootsweb.com.