A Segment-ology TIDBIT

I’d like to encourage all “Segmentologists” to periodically review the comments to my blog posts. I try to respond to every one of them and often go into more detail and/or provide suggestions for specific issues. If I may say so, there are often some more gems in the comments – including feedback from followers of this blog. I recently got a comment and elaborated on a post from over 7 years ago…

[22BK] Segment-ology: Review the Comments TIDBIT by Jim Bartlett 20221213

The question came up about siblings sharing the same crossover points. The answer is yes – some of them will be the same. Let’s look at this generation by generation. [There is often good discussion in the Comments to these blog posts – we are all learning on this journey. As a result of a recent comment, I decided to do a blog post about this topic]

The set-up:

1. One genome – let’s use our Mother’s side – 23 Chromosomes

2. Assume the average of 34 crossovers per generation.

3. A crossover is the point where DNA changes from one grandparent to the other grandparent, when the mother recombines her two chromosomes into a new one to pass on to a child.

4. Crossover points are random.

Mother’s DNA already has crossovers created by many of her Ancestors. She will recombine the DNA from her two parents at 34 places over the 23 chromosomes, and pass these new chromosomes to a child. Note: this means usually 0, 1, 2 or 3 crossovers per chromosome (on average 1 per 100cM). Since these crossovers are randomly formed for each egg, it would be rare for any of her children to have the same crossover from her.

The 34 new crossovers created 34+23=57 segments. These 57 segments “cover” all 23 chromosomes, from beginning to end of each one. These 57 segments are from Mother’s parents – our grandparents. All the crossover points from recombination events in prior generations are fixed (static) in the two grandparent’s DNA.

Example: Mother’s paternal DNA on Chr03 – from 47Mbp to 123Mbp has a crossover point at 68Mbp. Each of Mother’s children who got her paternal DNA that included the point at 68Mbp would include that crossover point. Mother could pass a paternal Chr03 segment 47-83Mbp to one child and paternal Chr03 59-119Mbp to another child – both of these children would have the same crossover at 68Mbp.

Note: The 68Mbp crossover could have occurred at the great-grandparent generation, OR at some previous generation.

This is a good example of why Chromosome Mapping *by generation* is important. In general Segment Triangulation results in Triangulated Groups (TGs) from different generations of ancestors. The TGs are not all from 4xG grandparents, or any other specific generation. However, if you have the Common Ancestor (CA) for your TGs, you can easily build a Chromosome Map for different generations. In my case I have 372 TGs – I know the CA side and grandparent for almost all of them – they roughly “fit” into about 114 groups (representing my 4 grandparents on both sides) on my 45 chromosomes.

Bottom line: Siblings won’t (generally) get the same crossover points from their parents, but likely will share some crossover points from grandparents and more distant Ancestors.

[05F] Segment-ology: Sibling Crossovers by Jim Bartlett 3 Dec 2022