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DNA Despair!

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Re: DNA Despair!

Postby RedCarp » Sat Jan 25, 2020 3:23 pm

From my professional experience as a solicitor and social worker my feeling is that granddad was on the scene when father was conceived and granny rather “hoped” he was the father and the pregnancy rather escalated their living together. Subsequent children are more likely than not to be granddad’s.
In one rather sad case a mother assured me her husband was the father, when he queried his paternity after their relationship broke down ( motivated by the desire to avoid maintenance) “ because I was only a little bit pregnant when I met him”. True he was the only father the wee girl had ever known and he and his parents adored her, but that didn’t change the facts. Tragic.


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Re: DNA Despair!

Postby Guy » Sat Jan 25, 2020 4:28 pm

Mick Loney wrote:fhb,
Sorry, but I disagree! DNA is at the heart of genealogy, which after all, is all about tracing our genetic ancestors i.e. DNA.
The documented family is all well and good, and perhaps nice to know in the family sense, but at the end of the day, what is the point of moving backwards from these individuals when they are not really related to you?
One didn’t get ones pointed chin, blue eyes or ginger hair from them, so IMHO researching them is a complete waste of time genealogically speaking.
A good example is adopted children. They wouldn’t dream of tracing their adopted parents history, they want to know who their real parents were.


Sorry but in many cases the current generation may not have inherited any DNA from a high proportion of their distant ancestors, it is even possible not to have inherited any DNA from a specific grandparent though the odds are about something like 1 in 8.4 million.
Average chances of no DNA from one great, great, great grandparent: 1 in 8

I have helped adopted people trace both their adoptive family and their genetic family and others who were not interested in their genetic family but wanted to trace their adoptive parents families in homage to them.

I myself have been interested in and researching my family tree virtually all my life but I had no interest in recording my father's tree until after my mother died in 2004 because he abandoned us in 1955. I did however research the family of my half-brother's father who was married to my mother before my father came on the scene.
People do not always research family history for genetic purposes.

Cheers
Guy
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Re: DNA Despair!

Postby Robbie J N » Sat Jan 25, 2020 6:39 pm

Sorry, I would not usually comment on the science of genetics of family trees, but I saw some bad interpretations which I feel compelled to correct. Sorry if this detracts from the original poster's query.

Saying that there is a 1 in 8,388,608 chance ( 2^23) that you would inherit no genes from a particular grandparent is only taking into account 1 of the 2 processes that occur before/during/after fertilisation that randomises the genetic material we get from each parent.
Guy had only taken into account the process of 'independent assortment' which on its own would give those odds he stated.
But there is also the process of 'crossing over', where chromosomes break, swap bits with the other chromosome in the pair, and then reform as 2 new chromosomes each containing material from both parents.
Over many generations genes from a particular ancestor can be lost through these 2 processes, but not as quickly as the 2 generations from grandparents.

Sorry to go all scientific and technical, but I studied this with the OU a few years ago and thought it needed correcting. It was a simple Level 1, 10 CATS point course called SK195 - Human Genetics And Health Issues.

The Y chromosome does not undergo 'crossing over' as it is not part of a pair of equal size chromosomes, hence why it is tested separately. The X chromosome only undergoes 'crossing over' in females, as they have 2. Mitochondrial DNA is different again, with no 'crossing over', hence a separate test for that too.

I apologise if this is getting off topic from the original poster's query.
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Re: DNA Despair!

Postby Guy » Sun Jan 26, 2020 9:03 am

Robbie J N wrote:Sorry, I would not usually comment on the science of genetics of family trees, but I saw some bad interpretations which I feel compelled to correct. Sorry if this detracts from the original poster's query.



I did broach on that subject by showing the very long odds against such loss of all DNA from a specific subject over a low number of generations.

To accurately display all the various combinations of possibilities would require a thread or possibly even a number of threads and even then I am sure many would still be confused.
Even then it would be possible for a single mutation or change to prove everything said previously understood to be fact to be inaccurate.
That is one of the things that makes science interesting.

Cheers
Guy
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Re: DNA Despair!

Postby Robbie J N » Sun Jan 26, 2020 12:04 pm

To Guy, sorry but I cannot quite tell if you are standing by your original statement or not, with regard to someone having a 1 in 2^23 chance of not inheriting any genetic material from a particular grandparent.

Using probability, statistics and permutations/combinations will give you chances only based on the process of ‘independent assortment’. That is not the only process that occurs.
The process of ‘crossing over’ is random, to an extent, and therefore cannot be calculated exactly. Breakages can occur along the chromosomes in random places, although always the same places for a particular pair so that they can recombine. Multiple generations of these 2 processes can eliminate an ancestor’s genetic material, but not in just 2.

During meiosis, the stage where the 23 pairs separate into 2 sets of 23 single chromosomes, the autosomal chromosomes can break apart and will swap segments between the 2 chromosomes in the pair, so that each of the chromosomes contains genetic material from both the mother and father of the person who is producing these cells. Therefore when this cell is involved in fertilisation, every autosomal chromosome will have parts from the 2 grandparents, the parents of the ‘donor’ of that cell. For example, a particular chromosome might contain 60% material from the donor’s mother and 40% from the father, while the other chromosome from the pair, which would end up in a different cell, would have those numbers the other way round. Another cell that splits in meiosis could have those same chromosome pairs ending up with a 70:30 split of mother to father genetic material instead. The exact percentage ratios and where the differences occur along the length of the chromosome amount to much, much higher possibilities for the number of different cells than just 2^23. (My textbook even says there are an ‘infinite’ number of combinations, though chromosomes are of finite length.)
The 2 generations from grandparent to grandchild are not enough to remove all genetic material from a particular grandparent.


Apologies to ‘meekhcs’ if this is of no interest or relevance to your circumstances.
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Re: DNA Despair!

Postby meekhcs » Wed Jan 29, 2020 4:14 pm

Robbie Guy et al
Interesting to read your observations.
I am not a DNA expert but have quickly learned the basics.
I have to say neither my Dad or I or my children have any resemblance to my “new Grandad’. However I look very much like my Aunts, Dad’s younger siblings, and there is a resemblance between us and the female descendants of my new Grandad.
I imagine none of my DNA matches will look normal because we all inherit different bits of DNA from parents etc. because maternal and paternal 5th gt grandparents are now the same people!...and because there has been lots of intermarriage in different generations and both my maternal and paternal lines contain the same surnames from the same places!
So I will continue to build a research tree and add in as many DNA matches as possible so that if nothing else it may help other people.
Thank you everyone for your contributions


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Re: DNA Despair!

Postby CaptainCaptain » Mon Feb 03, 2020 4:30 pm

Thanks for the thread. Much useful info here
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