# Sucesse rate of Egyptain brother/siser marriage



## Rex Mundi (Feb 6, 2017)

Several Egyptain royal family practices brother/sister marriages. One royal line was able to maintain this practice for 16 generations before they were unable to produce both a son and a daughter to continue the practice. 

Statictically, How may children woud need to be born in each generation to have a 95% probability that both a male and a female are available to continue the practice? Given an infant mortality rate of 50% how many children are need to assure the same 95% probability?


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## Michael K. Eidson (Feb 6, 2017)

You might be better served to go to Mathematics Stack Exchange to ask this question.


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## CupofJoe (Feb 7, 2017)

You would also have to remember that they would want only one female and one male child to make it to adulthood [or be old enough to make more children]. Not many things worse for a peaceful dynastic succession than a few rivals.


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## Vaporo (Feb 7, 2017)

It's a pretty simple problem. 1-50%^(6-1) = 96.875% chance of having both a male and female child after having 6 children. Because of the 50% mortality rate, double that to 12 children and you have your answer.


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## Alyssa (Feb 7, 2017)

Vaporo said:


> It's a pretty simple problem. 1-50%^(6-1) = 96.875% chance of having both a male and female child after having 6 children. Because of the 50% mortality rate, double that to 12 children and you have your answer.



You need to remember recessive alleles as well though. In cases of inbreeding you'll end up with varying combinations of homozygous pairs of 0-300 recessive alleles of various levels of deleteriousness. In a large gene pool you'll generally only share a couple of these with your partner. However, inbreeding leads to a concentration of these recessive alleles as the inbreeding continues down generations. One pharaoh (Ramses II) had about 100 children and I'm not sure how many were alive and fertile past puberty, but I suspect this pharaoh in particular was near the beginning of his lineage. Inbreeding and the concentration of recessive alleles is part of why so many Amish have polydactyly.


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## TheCrystallineEntity (Feb 7, 2017)

Now I remember one of many reasons why I decided that all of my characters, regardless of what book they are in, reproduce by energy fusion.


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## Vaporo (Feb 7, 2017)

Alyssa said:


> You need to remember recessive alleles as well though. In cases of inbreeding you'll end up with varying combinations of homozygous pairs of 0-300 recessive alleles of various levels of deleteriousness. In a large gene pool you'll generally only share a couple of these with your partner. However, inbreeding leads to a concentration of these recessive alleles as the inbreeding continues down generations. One pharaoh (Ramses II) had about 100 children and I'm not sure how many were alive and fertile past puberty, but I suspect this pharaoh in particular was near the beginning of his lineage. Inbreeding and the concentration of recessive alleles is part of why so many Amish have polydactyly.



Yes, but there are a lot of variables at play there depending heavily on the genetic history of the initial couple. If you want to learn about the affects of inbreeding on fertility rates, your best bet would probably be to research cases of extreme inbreeding throughout history and see how many generations it lasted and why it ended.

I guess the best answer to the original question is "as many as possible."

Also, would the royal family be willing to secretly adopt children if they saw the affects of such extreme inbreeding? That could make for an interesting plot line.


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## Alyssa (Feb 7, 2017)

Vaporo said:


> Also, would the royal family be willing to secretly adopt children if they saw the affects of such extreme inbreeding? That could make for an interesting plot line.



This. XD
So much this.


Sent from my iPhone using Tapatalk


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## DragonOfTheAerie (Feb 8, 2017)

You might want to consider whether these horribly inbred heirs will be fit to rule. If too many deleterious effects pile up, your rulers won't be capable of ruling/hanging onto the throne. So the line might end before it becomes infertile.


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## Insolent Lad (Feb 12, 2017)

A variable here is that the society might permit polygamy and the male ruler could marry more than one sister, increasing his odds. I don't know if the Egyptians did this but it occurred in some other cultures that practiced sibling marriage.

We should also consider that women are not always faithful to their mates and other genes might get into the mix with no one the wiser.


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