First cousins

[jakethebaptist]

Many ancient societies practiced sibling marriage among the rulers. This usually is not documented to have any issues for numerous generations. Once the genetic inbreeding became apparent, the practice is normally seen to drop off, though it in some cases it is restored after a few generations.

Biblically, Abraham and Sarah were half-siblings. Isaac and Rebekah were first cousins. Jacob was first cousin to Rachel and Leah.

The nations of Ammon and Moab resulted from an incestuous father-daughter relationship. They turned out to be large, powerful nations for several centuries. It is from the Moabites we get Ruth, close ancestor of David and eventually Jesus Christ.

First-cousin marriage was universally accepted, and in many cases even encouraged until the 20th century. Even still, it is an acceptable practice in most of the world today, and actually legal in a significant number of states in the USA. Recent studies have shown that the real chance of increased genetic birth defects from first cousin marriages is so insignificant that it is a non-issue.

Objections to the necessity of sibling or cousin marriages in early Genesis are unfounded, even without going into the deeper scientific genetic studies.

 

[atpollard]

Genetic Risk [just FYI]:

"All of us have some recessive disease genes. But we usually have a second, working copy of that gene to make us healthy. When you have one normal copy and one disease copy of a gene, you are called a "carrier" of the disease. Carriers don't show signs of the disease, but they can pass the disease version of the gene to their kids. However, BOTH parents must be carriers in order for their children to have a chance of getting the disease.

There are many different recessive diseases and they tend to be rare. What this means is that two unrelated parents are most likely carriers of different disease genes from each other. If they have kids together, the kids will probably get at least one good copy of any one of those genes. And so the kids are unlikely to get those diseases.

On the other hand, siblings are very closely related. So they are much more likely to be carriers of the same diseases. And their kids are more likely to get two broken copies of those genes and end up with the disease.

But how much more likely? It turns out that two siblings are 50% related. This means that for any given gene there is a 1 in 4 chance that they have the same copy as each other.

Say their dad (but not their mom) is a carrier for a harmful disease such as cystic fibrosis (CF). So dad has one broken copy of CFTR, the CF gene. This means that the brother and sister have a 25% chance of both also being carriers.

If the brother and sister are both carriers and have a child together, then each of their children would have a 1 in 4 chance of ending up with CF by getting a disease copy of CFTR from each parent. So the odds of this brother and sister having a child with the disease is (1/4)(1/4) or 1 in 16.

Now imagine that dad is a carrier but that his kids go on to have children with unrelated people. What are the odds that these grandkids will have CF? Around 1 in 240.

We can calculate this number because we know how likely it is for any random person in the U.S. to be a carrier of CF--around 1 in 30. We also know that because dad is a carrier, each of his children has a 1 in 2 chance of getting the disease version of the CFTR gene. So the odds that both parents would be carriers is (1/2)(1/30) or 1 in 60.

The chances that both of these parents would then pass on the disease version of CFTR is the same 1 in 4 that we talked about in our previous example. So the chances are (1/4)(1/60) or 1 in 240.

So the difference between our two examples is 1 in 16 vs. 1 in 240. The siblings are 15 times more likely to have a child with CF than if they had had kids with an unrelated person. The risk becomes more pronounced with more rare diseases.

Imagine a really rare disorder like complete achromatopsia, or total color blindness. Somewhere between 1 in 20,000 and 1 in 50,000 people have this disease. This translates to around 1 in 100 people being carriers.


If we use the same example where dad is a carrier for the disease gene, then the chances for two siblings having a child with the disease are the same, 1 in 16. But the risk for one of these kids and an unrelated parent falls to 1 in 800. So the siblings have a 50-fold higher risk!"

- from HERE

So 15/16 (94%) of the sibling marriage children will be just fine.