DNA Shared by Relationship

From:  https://customercare.23andme.com/hc/en-us/articles/212170668-Average-percent-DNA-shared-between-relatives

Interesting that 47.5% of DNA is shared in a father son relationship and not a full 50%. I read once that the weakest relationship between a grandparent and grandchild is between a boy and his paternal grandmother, because a male child will never inherit any of the X DNA from his father’s mother.

It seems incorrect that two first cousins share half of their ancestors but only an eighth of their DNA. But that's a result of every child inheriting a random half of the DNA from each parent.

Not really half, unless you count the in-laws.

Here is an interesting article:

https://historyofyesterday.com/einstein-darwin-both-married-their-cousins-ec6adedebde8

I knew that Darwin married his first cousin but didn’t know that Einstein also did the same.

Even with 12.5 percent of shared DNA, it’s still not a good idea genetically speaking.

I think it's right to discourage cousin-cousin marriage, although the risk of producing genetically disabled children is not as high as many might think - LINK:

Unrelated: 2.5% risk (1 in 40)
2nd cousins: 3.5% risk (1 in 29)
1st cousins: 5% risk (1 in 20)

When I say two first cousins share half of their ancestors, I mean they have thee same grandparents on one side and therefore all the older relatives back through time on that side too.

When you look at the statistics, the risk for first cousins having a child with a disability is probably not much higher then for children born to older parents, (even still there is the ick factor).
The children however, as Darwin suspected, may not be as biologically fit as offspring from parents that are not so closely related.
Of the ten children born to Darwin and his first cousin Emma Wedgewood, three died in childhood, of the six surviving, three remained childless despite being in long term marriages.
Inbred depression, can lead to decreased fertility in the children of closely related parents.
https://en.wikipedia.org/wiki/Inbreeding_depression
There is also the phenomenon of outbreeding depression, where the offspring of people that share little DNA are said to not be as genetically fit. It is much harder to find information and data on this, as it is said to be a racist theory.

This relates (no pun intended) to the last thing you said:
http://www.scientificamerican.com/article/when-incest-is-best-kissi/

Women these days are having less children, you just don’t see big families anymore. It’s kinda sad.
I am Rh negative and am mother to three Rh positives babies. I was very lucky that Rhogam was available to me. I can’t imagine the anguish of over and over loosing pregnancies and babies and not even knowing why.
When my mother (A neg) was having her children, there was no medication but she was lucky too because, I was the first born, then a positive baby sister and the last one another negative girl.
They say that only 6% of the world’s population is Rh negative, if half are male how many of those men would be eligible do you think. . . blood compatibility is not everything. You have to factor in age, homosexuality and jerks.
If a negative woman was looking exclusively for a mate with  her own blood type, it would really narrow down her choices. She might even end up an old maid.
Now, Rh negative men on the other hand, have no problem at all with blood type incompatibility and can have healthy  children with either negative or positive women. It doesn’t seem fair somehow, but I guess that’s what makes us negative women a little different.
A bit of an enigma, you might say a subspecies all of our own.

I learned that one of my second cousins played a major role in the King's recent coronation procession due to his position in one of the armed forces. I've heard his name mentioned occasionally but I don't know if I've even met him - if I have it must have been when we were both young children. How well do you know your second cousins (the children of your parents' first cousins) ?

The same as you, I remember meeting four of my second cousins, my father’s cousins children, but that was a long time ago. All of my mother’s aunts and uncles stayed in the old country.
I have 19 first cousins, most of them are now spread out across the United States and I keep contact with only two. It seemed that once my father and grandmother died we drifted apart. I sometimes wonder if I would even recognize the ones that I knew and played with when we were children.
I checked 23 and me and I have 7 second cousins listed. One of them I know, she is my first cousins daughter and we have the strongest match with 6.9% shared DNA.

That is very cool, your cousin playing a role in the coronation. It is an honor to your living family as well as your ancestors.

Sary wrote:Women these days are having less children, you just don’t see big families anymore. It’s kinda sad.
I am Rh negative and am mother to three Rh positives babies. I was very lucky that Rhogam was available to me. I can’t imagine the anguish of over and over loosing pregnancies and babies and not even knowing why.
When my mother (A neg) was having her children, there was no medication but she was lucky too because, I was the first born, then a positive baby sister and the last one another negative girl.
They say that only 6% of the world’s population is Rh negative, if half are male how many of those men would be eligible do you think. . . blood compatibility is not everything. You have to factor in age, homosexuality and jerks.
If a negative woman was looking exclusively for a mate with  her own blood type, it would really narrow down her choices. She might even end up an old maid.
Now, Rh negative men on the other hand, have no problem at all with blood type incompatibility and can have healthy  children with either negative or positive women. It doesn’t seem fair somehow, but I guess that’s what makes us negative women a little different.
A bit of an enigma, you might say a subspecies all of our own.

I'm Rh negative as well, as are all my siblings. My parents are both Rh negative. I have a long line of people in my family who had large families and "fell in love" easily. I also have two children who are Rh negative, and I suspect that his parents (my husband's and therefore my husband) are Rh negative, else how would you explain my children?

His parents, like my parents, basically fell in love at first sight. They were a little more dramatic than mine, as they ended up eloping within two weeks of meeting each other. I have few infant deaths stretching back two hundred years in my family tree, which is weird given how rustic they lived (living in small villages of Cuba, Asturias and Canary Islands, Spain). None of my ancestors within the last two hundred years were upper class or middle class in any way, most were respectable farmers but I do have some sailors, ne'er do wells and ruffians who weren't inclined to care for their children, and yet most survived anyway.

I guess what I'm trying to get at, is that there has to be a biological process that we aren't aware of that has kept this particular trait from becoming extinct. Perhaps scent?

As for the topic at hand, I don't have a problem with cousin marriages as long as they are third cousin and above. I actually think that's been the norm throughout human society, stretching back to the neanderthals before they were isolated. First cousin marriages aren't that bad as long as they aren't generational.

One of my set of great-grandparents were said to be first cousins, but since this particular line was the least educated and the man in question was the ne'er do well, I have no idea if they were actually first cousins or second. I do know that they had seven surviving children of eight, my grandmother being the second born but first surviving, and that none of them were considered disabled in any way. My grandmother was also beautiful and intelligent, and married above her class. The one peculiarity about that side was that they were very superstitious and had problems with mental illness. One of her sisters committed suicide at the age of eighteen through self-immolation.

Oh wow OP you are Rh negative too!  That is very interesting seeing that negative blood is somewhat rare . . . let me guess A- like me

I can sometimes recognize other Rh- people and if I feel comfortable enough, I will broach the subject and ask.  I am usually correct. I can’t say that I sense anything different about our smell, it is more like a feeling that i get, an unspoken understanding. We can be quite peculiar, myself included.

There is some data that suggests that rh negative blood has not become extinct because  carrying the negative gene recessively gives a heterozygous advantage I.e hybrid vigor.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4728066/

Sary wrote:Oh wow OP you are Rh negative too!  That is very interesting seeing that negative blood is somewhat rare . . . let me guess A- like me

I can sometimes recognize other Rh- people and if I feel comfortable enough, I will broach the subject and ask.  I am usually correct. I can’t say that I sense anything different about our smell, it is more like a feeling that i get, an unspoken understanding. We can be quite peculiar, myself included.

There is some data that suggests that rh negative blood has not become extinct because  carrying the negative gene recessively gives a heterozygous advantage I.e hybrid vigor.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4728066/

Yes, I'm A-, my daughter is O-, my son is also A-. I suspect my husband is O-.

OsricPearl wrote:

Sary wrote:Oh wow OP you are Rh negative too!  That is very interesting seeing that negative blood is somewhat rare . . . let me guess A- like me

I can sometimes recognize other Rh- people and if I feel comfortable enough, I will broach the subject and ask.  I am usually correct. I can’t say that I sense anything different about our smell, it is more like a feeling that i get, an unspoken understanding. We can be quite peculiar, myself included.

There is some data that suggests that rh negative blood has not become extinct because  carrying the negative gene recessively gives a heterozygous advantage I.e hybrid vigor.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4728066/

Yes, I'm A-, my daughter is O-, my son is also A-. I suspect my husband is O-.

If your daughter is O- then you must be A/O- and carry the O- gene recessively, the same as me.  My children are all O+.
It’s hard to know your husband’s Rh status from your childrens type. He could potentially be A+/O-, A-/O-,  O+/O- or O-/O-.
You could always buy an Eldon blood type kit on Amazon if you are really curious.  They are pretty cheap, about ten dollars.
I tested my kids, and have to admit I was a little disappointed that none of them carried two recessive genes like me.  I’ve known my own type because I use to donate blood and when I was pregnant they pumped me full of rhoGAM.