Surnames and illegitimacy - is my math correct?

[melba_schmelba]

I feel I should confess that I have no idea whatsoever what you are on about reading the varying DNA tests you mentioned lol I have been trying to find out information about the birth mother of my maternal grandmother who was illegitimate and adopted aged two

In your case a standard (autosomal) test as Ancestry or 23andme offer would be your best bet for that Joby , then upload the tests to MyHeritage, FTDNA and GEDMATCH, and on GEDMATCH you could combine the two kits for better accuracy.

[Joby86]

You sound amazingly intelligent and I love the idea that I am of a decent standard of intelligence but times like these are quite sobering lol is your knowledge based on self teaching or are you as I expect some form of scientist?

[Andrew Tarr]

Ah, but I am not talking about the probability of each event individually, I am talking about the accumulated probability over many generations that at least one ancestor in the male line will have been illegitimate .

As well as the illegitimate ones - born of an unmarried mother - don't forget the others born of a married mother but an unrelated father  So after 10 generations I guess a genuine family line as documented may be pretty rare ....

[melba_schmelba]

You sound amazingly intelligent and I love the idea that I am of a decent standard of intelligence but times like these are quite sobering lol is your knowledge based on self teaching or are you as I expect some form of scientist?

LOL no Joby, just a genealogy geek  who has learnt from forums like this, blogs, vlogs, Rootstech, and yes, even books . I am sure some on here are far more qualified as far as mathematics are concerned.

[melba_schmelba]

Ah, but I am not talking about the probability of each event individually, I am talking about the accumulated probability over many generations that at least one ancestor in the male line will have been illegitimate .

As well as the illegitimate ones - born of an unmarried mother - don't forget the others born of a married mother but an unrelated father  So after 10 generations I guess a genuine family line as documented may be pretty rare ....

Well yes exactly  . So even after 150 years, there is probably a 25% chance that your male line originated in someone with a different surname, so it really is going to be something quite common for a lot of the population to potentially find out when doing DNA testing and genealogy and comparing your DNA test to someone of the same surname, someone you suppose to be a 3rd cousin but DNA suggests are not.

[Joby86]

I can't help thinking that the maths is adding to confusion but I have that thought possibly because I may misunderstand the point of the illegitimacy rate compared to surnames which is of course down to moi! Lol
I am very new to all this stuff but with politeness, charm and whatever else I can conjure up.... I hope people will take pity and help me lol

[~buttons~]

If the supposed average rate of illegitimacy is 5% - does that mean, in 10 generations i.e. on average, about 300 years, the likelihood of your ancestor in the male line being someone of a different surname is 50%? And in 600 years, it is almost certain that one of your male ancestors would have had a father who was not who he was meant to be (or they inherited their mother's surname) ?

An average of 5% remains as 5% irrespective of the number involved.

The base number might change but an Average percentile remains static

[Andrew Tarr]

I can't help thinking that the maths is adding to confusion but I have that thought possibly because I may misunderstand the point of the illegitimacy rate compared to surnames which is of course down to moi!

The illegitimacy rate should indicate 'births out of wedlock', or Base-born as cantankerous clerics used to write.  Some of those (quite a few today) may have the true biological father, the parents having regularised the situation later.  There is one on my wife's tree, who was subject to a bastardy order, but the father came round and made an honest woman of the mother.

[melba_schmelba]

If the supposed average rate of illegitimacy is 5% - does that mean, in 10 generations i.e. on average, about 300 years, the likelihood of your ancestor in the male line being someone of a different surname is 50%? And in 600 years, it is almost certain that one of your male ancestors would have had a father who was not who he was meant to be (or they inherited their mother's surname) ?

An average of 5% remains as 5% irrespective of the number involved.

The base number might change but an Average percentile remains static

I don't think this is correct. Think of it this way......

there is a game in which you have a 5% chance of winning - if you play once, you have a 5% chance to win, but if you are allowed to pay twice, you have a 10% chance you will eventually win, play four times, a 20% chance etc.

Similarly if you are baby that is born, there is 5% chance your father will not the person whose surname you bear. If you think of each birth as a separate but linked event, the chance increases the number of 'tries' at this game that occur.....