First 1st Cousin DNA Results: Part 2 – Trying to Explain Aunt/Nephew Matches

First, Another Look at First Cousins

In my last Blog I took a first look at my 1st cousin, Rusty’s DNA. I went into some detail on how he matched on a few of the lines we have in common. I looked at how Rusty compared to me and my siblings on Chromosome 16. Here is a visual summary of that comparison:

The first image is a chromosome map of the DNA that my 3 siblings and I got from our four grandparents. The red and yellow grandparents are the maternal ones shared with Rusty. The second image shows Rusty’s matches with me and my 3 siblings. Note that the long segments shared are similar to the Lentz segments on the left and the Rathfelder segments on the right. Note that as Rusty and my siblings are of the same generation, we share the same long segments with our grandparents. From this, I was able to create a maternal Chromosome Map for Rusty.

As there were a lot a matches, I would assume that the DNA profiles of my mom and Rusty’s mom were somewhat similar to each other on Chromosome 16.

Rusty Compared to His Aunt – My Mom

In the above example, the common ancestors of Rusty and me are our two maternal grandparents. When I compare Rusty and my mom, I will be looking at two different generations.

Here Rusty and my mom share the same common ancestors as me and my mom. However, do Rusty’s and my mom’s shared segments represent my mom’s parents’ or my mom’s grandparents’ DNA?

Does rusty share DNA with my mom’s grandparents (his maternal great grandparents)?

My thinking is that when I compare Rusty to my mom the DNA compared goes up a generation from when I compare Rusty to myself and my siblings. Here is Rusty again at Chromosome 16 compared to my mom:

My mom has a full Lentz and a full Rathfelder Chromosome from her parents. Yet there is a place in the middle of Chromosome 16 where Rusty and my mom do not match. That makes me think that we are comparing my mom’s grandparents with Rusty’s great grandparents. Let’s assume that to be the case. That means we need to bring in another generation.

With what we know of Chromsome 16, Rusty and my mom must share all of the same Lentz next generation up. That would be either Jacob Lentz or Annie Nicholson. The same must be true for the Rathfelder side from position 56M to 88M. However between about 50M and 56M Rusty and my mother must get their DNA from different paternal grandparents of my mom.

a look at chromosome 10

Here is the way I have mapped my mom’s chromosomes using Kitty Munson’s Chromosome Mapper:

The DNA match in purple is my from my mom’s Nicholson only side. It is mapped to William Nicholson and Martha Ellis who were the parents of my mom’s grandmother Annie Nicholson.

First, let’s look at my mom’s matches on Chromosome 10. I had discussed this Chromosome in my previous blog also.

#1 is mom’s match with Carolyn which maps to the Nicholson side. #2 is mom’s match with Rusty. #3 is a small match with Catherine which I’ll ignore for now.

Here are Rusty’s matches:

#1 is Carolyn. #4 in my mom. 2, 3, 5 and 6 are me and my siblings – not so important for this comparison. #7 is Linda (Nicholson descendant) and #8 is Catherine (Rathfelder descendant).

A possible explanation of a maternal aunt/nephew match

I have to admit that this gets a bit confusing.

When we compare my mother to Rusty, we are looking at my mom’s maternal and paternal chromosome. However, the match to Rusty is all on his maternal chromosome. Conceptually, I think that it would look something like this.

The top showing my mom has her 4 grandparents on her maternal and paternal chromosomes. I don’t know how my mom’s paternal side might look, so I made something up there. My mom’s four grandparents are equivalent to Rusty’s 4 great grandparents, but those 4 great grandparents are all on Rusty’s maternal Chromosome. So they are cramped in to a smaller space.Said another way, Rusty’s maternal DNA is alternating between Rathfelder and Lentz. However, that Rathfelder grandparent may be broken up further to two great grandparents of Rathfelder and Gagnus. Likewise the Lentz grandmother may be broken up to Lentz and Nicholson great grandparents.

In the first segment, my mom has Nicholson DNA due to the match with Carolyn. Rusty has a Rathfelder match in that segment. However, as my mom doesn’t also match Catherine in that segment, it must be from a different Rathfelder. My mom’s grandparents were Rathfelder and Gagnus. So here my mom has either one of those grandparents’ DNA and Rusty has the opposite. That is why I have blue for my mom there and green for Rusty.

A final note is that the last small segment match that Rusty has with Catherine cannot be right. Or it cannot be Rathfelder. That is because Rusty’s DNA is alternating¬† between Rathfelder and Lentz. The last segment has to be Lentz, so there is no room for Rathfelder DNA there. On the other hand, my mother’s #3 match is with Catherine, which is a Rathfelder match. She has room for that match along with her Nicholson match as she has a maternal and paternal chromosome to match on.

Summary

  • In a 1st cousin match, the DNA from my two grandparents are compared to the same DNA that my first cousin got from those same two grandparents
  • In a nephew/aunt match, the great grandparents of the nephew are compared to the grandparents of the Aunt
  • The aunt, however, has her 4 grandparents’ DNA on 2 chromosomes
  • The nephew has his 4 great grandparents’ DNA On only one chromosome
  • Those 4 great grandparents have to fit within the appropriate alternating grandparents of the newphew

The Segmentology Blog, Segments: Bottom-Up explains it well. Here is an image from that Blog:

In my example above, this Segmentology image would be like Rusty’s maternal DNA. In Rusty’s grandparent look, his maternal DNA alternates between Rathfelder and Lentz. However, in his great grandparent look, the DNA may be split up between the parents of those grandparents within the crossovers of the grandparent look.

For my mom, I am just looking at her grandparents. However, there will be two lines of grandparents: maternal and paternal for her. Also the crossover points will in most cases be different than for Rusty as he got his DNA from his mom – my mom’s sister.

 

 

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