A Segment-ology TIDBIT
A question recently came up: Are the Ancestors on two sides of a crossover point, always a mother and father (in either order)? Or: If I know the Common Ancestor (i.e. the father or the mother of the TG couple) of a TG segment, must the next TG segment be the other parent of the TG couple.? The answer is YES, with an important caveat: only when we are talking about mother and father of our Ancestor who created the crossover.
Important scientific fact: A crossover is formed when a human recombines two Chromosomes to create a new Chromosome that is then passed to a child. One of the two Chromosomes is from the Mother, and the other is from the Father. So one parent is on one side of each crossover, and the other parent is on the other side of the crossover. Here is Figure 6 from my 2015 blogpost: Segments – Bottom Up:
Note: each of the Chr 05 lines above is your Maternal Chr 05 – it’s just broken down for each generation. In the Grandparent look, the two crossovers were created by the parent using grandparent segments (assuming an average of 2 crossovers per generation for Chr 05). Note the Ahnentafel numbers to represent generic ancestors – even numbers are males, odd numbers are females. The first crossover created by the parent shows 7 & 6, or female & male, on the two sides of the crossover. When the first grandparent segment ends at the crossover, the next segment is the opposite parent. The second crossover created by the parent has 6 & 7 (male & female) on the two sides of the crossover.
The next line – the Great grandparent look has 2 more crossovers – created by the grandparents, when each of them recombined their respective 2 Great grandparent chromosomes. One of the crossovers is between 14 & 15 and the other between 13 & 12 (there was no crossover when the Ancestor 14 segment was passed to daughter 7). So again, each new crossover has a male and a female (in some order) on the two sides of each crossover.
Check out the two crossovers (on average) added at each of the next two generations – they all have the mother on one side and the father on the other side of the crossover. Note carefully the word “added” (or created or formed).
Now here is the catch… In the Great grandparent look above, the last crossover has 12 & 14 on each side – two males. This seems to contradict the basic concept. And if we were applying the basic concept to TGs at the Great grandparent level it would be wrong. What’s up? Well, what looks like a crossover between Ancestors 12 & 14 is in fact a crossover – but it was formed by Ancestor 3 when she recombined Chr 06s from her parents 6 and 7 – these are the two parents of the ancestor who first formed (or added or created) the crossover.
When we form Triangulated Groups (TGs), we use groups of overlapping segments. But there is nothing in the TG criteria about the generation of the TG. We do understand that the TGs start and end at crossover points – when we shift from one Ancestor’s DNA to another Ancestor’s DNA. But until we can Walk the Segments Back (generation by generation), we don’t know when the crossovers were formed. There is one generation for each crossover, but until we have Chromosome Mapping we don’t know which generation it is.
Note: A TG Summary Spreadsheet will give good clues to the formation of crossover points – see Observation 5 (see linked blogpost). In generation after generation the older crossovers can be seen, with only about 2 new crossovers in each generation. So the farther back we go with Chromosome Mapping, the newly formed crossovers will be there (with mother and father on the two sides). But the other crossovers may not appear to be mother/father, unless the origin of the crossover can be determined.
Bottom Line: With TG segments, sometimes the next TG on a chromosome will be the other parent, but more often it will not.
Edit 20240403: It was suggested that I add a Chromosome Map, showing segments from my 16 2xG grandparents. Here is one I did in 2013:
[22CA] Segment-ology: What is the Next Segment? TIDBIT by Jim Bartlett 20231209
segment-ology 
Hi Jim, thank you for all you do.
Would you be able to post an anonymized example of how the 3GGparents segments are laid out on the chromosomes. i.e. 32 people, 16 couples, with the couple pairings identified ?
If would be informative to see a real-world example of how each person is represented and how often the couple segments do end up adjacent over the 22 Chromosomes.
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Thank you Jim.
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Ciao jim kevin.ho visto una corrispondenza di quel gruppo che ti parlo sempre ha un altro segmento sul chr 4 e stamattina ho ricevuto un match con15,7cm di segmento e lui e patetno sicuro e lui ha segmento sul chr 4 che si sovrappone al match sconosciuto ma non triangola allora ha questo punto e materno quella corrispondenza che fa parte di quel gruppo misterioso.perche ho anche una corrispondenza da russia con segmento che 10,5cm e stessa posizione dei 16 di quel gruppo sconosciuto ma non triangolano.ma questo match da russia non so da che parte e .pero questo che ho avuto stamattina ha stessa direzione sul chr 4 con uno del gruppo che ti parlo sempre.ma non triangolano questo di stamattina e paterno si sovrappone sul chr 4 di questo corrispondenza di quel gruppo sconosciuto allora e materno pure quei sconosciuti sul chr 16.faro testare mia madre e poi mio padre cpsi tolgo il dubbio.perche ho poche match itali 208 su 885 match la maggior parte sono del sud italy come me e altri sono maggior parte europa ma piu dell est. Il 10 dic 2023 7:30 PM, “Kevin Capodivento” capodiventokevin6@gmail.com ha scritto:
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Kevin.grazie per i consigli molto precisi.ma voglio dire quel gruppo e un po strano che l ultima corrispondenza era un mezzo cugino di primo grado di un altra corrispondenza e poi ci sono io in mezzo che loro sarrbbero due figli di cugini di quaryo grado.poi ho un altra corrispondenza che e mezzo cugino di primo grado come i primi che ho detto e anche qui ci sono io e ancora un po strano poi una corrispondenza ha mezzi cugini di primo del genitore poi ha cugini di srcondo e terzi cugini ecc ma tra i suoi cugini di mezzi primo secondo e terzo ci sono io un po piu in la ma e strano che queste persone si fanno test e gi puo darsi che lo sanno che loro tra loro sono cugini di 1 2 3grado e poi ci sono io ch per loro sono l unico italiano diciamo e molto strano.io li ho contatati due di loroma non sanno niente poi i loro alberi non hanno molto su myheritage .
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Another great post Jim, thanks. I often talk about subgroups within a TG when looking at close cousins trying to explain that they could relate to different distant ancestors on multiple lines. I love your figure 6 using ahnentafel numbers as explains it visually so much better!
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