Ken wrote: ↑Fri Jul 21, 2023 3:58 pm
Neto wrote: ↑Fri Jul 21, 2023 1:45 pmI do not recall now where I first read this, but it's really quite logical.
But I'll just quote from one source that touches on this question:
In other words, Mendelian inheritance principles show that parallel first cousins are closer genetically than cross first cousins. A parallel cousin is one from a parent's same-sex sibling, while a cross cousin is from a parent's opposite-sex sibling. It is a fact that “higher than normal consanguinity which means an increased chance of sharing genes for recessive traits. With a high amount of shared DNA, one has a higher risk of birth defects in a baby,” [16]. These statements are made with regards to a person, Male 3A, who marries his first cousin on the father’s side, Female 3A, for instance. It is a perspective in the current work, that birth defects from consanguinity would be reduced if first cousin marriages shifted from Male 3A-Female 3A marriages, to Male 3A-Female 3A’ marriages. In other words, birth defects from consanguinity would be reduced if first cousin marriages took place between cross cousins rather than with parallel cousins. The genetic risks of recessive genes manifesting themselves in the babies are reduced, while wealth could still be preserved in the family.
https://www.researchgate.net/publicatio ... f/download
Your source says the same exact thing that I said. They are only talking about X-linked genes:
In the first cousin level, there is expected to be differences in the X-linked genes in the Male 3A and the Female 3A’. (At the second cousin level, the expected differences in the X-linked genes in the Male 4E and the Female 4D’are even greater than those of their respective parents, the Male 3A and the Female 3A’.) On the other hand, there is less differences in the X-linked genes in the Male 3A and the Female 3C because the X-linked genes of Female 3C are pretty close to those of Male 3A’s sister, Female 3A. The salient point is that the expected differences in the X-linked genes in Female 3A’ are greater than those of Female 3A. In other words, Mendelian inheritance principles show that parallel first cousins are closer genetically than cross first cousins. A parallel cousin is one from a parent's same-sex sibling, while a cross cousin is from a parent's opposite-sex sibling.
What they are saying is that there are a subset of genetic diseases or recessive traits that are carried on the X chromosome such that you only inherit them from your mother if you are a boy but can inherit them from either your mother or father if you are a girl. So yes, parallel cousins are going to be more likely to share those traits than cross cousins. But that is only going to be the case for genetic disorders carried on the X chromosome.
For genetic disorders carried on any of the other 22 chromosomes there is going to be no difference between parallel and cross cousins since there is equal chance that you will get the trait from either your mother or father. So if I'm doing the math right, and if genetic disorders are evenly distributed across all the chromosomes, then you would be 1/23 more genetically linked if you are parallel cousins rather than cross cousins. That means that parallel cousins are 4% more genetically linked than cross cousins and only for traits on the X chromosome. I think the math is more complex than that, but that is the gist. It is a measurable difference but not a huge one. And only relevant for disorders carried on the X chromosome.
OK. I'm obviously not a geneticist. But the researchers who wrote that paper felt that it was/is important enough to suggest a far reaching cultural change, one that would bring about a severe cultural upheaval if the reverse track were pursued among the Banawa. Parallel-cousin marriages are not just discouraged, they are considered incestuous, the same as if an actual blood brother married his sister. It must have been enough of a difference that the Banawa somehow noticed the difference. (Of course no ethnographers were there observing the formation or development of the Banawa culture, and once formed it becomes extremely difficult to establish cause & effect.) So the current marriage pattern could be due to a higher prevalence of disorders linked to the X chromosome in their genetic background, OR it is also possible that it has nothing at all to do with genetics, but is a result of their emphasis on the brother-sister relationship. In this culture, a man never goes to visit his brother at his brother's home, nor does he ever share meat with his brother, because that would mean that he was providing food for his brother's wife, who was also culturally designated as a preferred potential spouse for himself. (In fact, the kinship term for a cross-cousin is 'other wife', or 'other husband'. And the kinship term for all of one's father's sisters is 'mother-in-law', and all of one's mother's brothers are called 'father-in-law'. It could of course be argued that the reverse is the case, that one's father's sister is called 'paternal aunt' or some such, and there are simply no distinctive terms that mean 'mother-in-law' and 'father-in-law'.)
But this is all a circular reasoning process, as either cultural rule could be a result of the other. That is, if the culture didn't favor cross-cousin marriage, but rather parallel-cousin marriage, then it would simply involve a different set of 'cousins' - in parenthesis because they do not recognize cross cousins as relatives at all; they are simply the preferred group from which to choose a mate, thus all referred to as 'wife' or 'husband'. However, the fact that there is no kinship term for one's same sex cross cousin suggests that the kinship classifications are not driving this distinction for a potential spouse. (I have always found kinship terminology of particular interest, so I explored this extensively, even though it was not at all necessary from the standpoint of Bible translation.) [One further comment, then I'll let it go: This distinction is not a matter of Patrilineality vs Matrilineality, because everyone has cross-cousins on both sides of their family - the men are not always choosing a wife from their father's family; they might just a likely chose a wife from their mother's family.]