What would the starting shared cM number be for abnormally close relationships in Autosomal DNA test?

Question

I have seen in the examination of family trees where individuals sharing a great grand parent marry and have children, somewhat commonly in pre-American Civil War, which I would think would create some interesting confusion when examining Autosomal DNA (atDNA) results?

I have reviewed ISOGG's Autosomal DNA results chart on what to expect, and reviewed the prior questions "Generational Loss with DNA testing" & "Working with Autosomal DNA results" but they do not address this or other close relationships (both illegal and taboo) on how that might look in an autosomal DNA result, knowing how this might 'look like' would be interesting reference information.

I will also first caveat that I realize that I am aware that more distant relationships will also influence shared cM results such as if you happen to share a relative 5 generations back and didn't know it. In this case I am looking for clarification on what the 'starting number' should be so I can use this for exploring theories about distant relationships as well as for 'orphan' situations.

Question: Has anyone seen a chart (or can produce one or explain) of how the following relationship's offspring would look like with a Autosomal DNA to a common individual?

Such Would you split the difference or still use the 50% rule to have the expected number of shared cMs?

• 2nd Cousins having a child
• 1st Cousins having a child
• Uncle/Aunt & Niece/Nephew having a child
• Brother & Sister having a child
• Parent & Child, having a child

In perhaps the easiest example the incest relationship of parent and child; assuming the child had the 3400 cMs of the involved parent (~50%) would their resulting child be higher than 3400 cMs of the involved parent or does other math come into play and by how much?

I am primarily looking for Autosomal shared cMs at this time vs. Autosomal and X, but if you can provide an explanation of X too great.

Answer 1

I don't have a direct answer to the question (yet), but you may be able to estimate it by using information found in discussions of GEDmatch's Lazarus Tool or other calculators.

• Lazarus – Putting Humpty Dumpty Back Together Again posted on January 14, 2015 by Roberta Estes. See the section Recreating Ferverda Brothers where Estes discusses the analysis of a reconstruction based on a person and two of her first cousins who are siblings.

• Recreating a Grandmother’s Genome – Part 1 Posted on 9 December 2014 by Blaine Bettinger

• Recreating a Grandmother’s Genome – Part 2 Posted on 14 December 2014 by Blaine Bettinger

All of these talk about the number of cMs you need for the different types of analysis offered by this tool.

Estes also has a tag of Ancestor Mapping and those posts have information which might be useful for your analysis.

• Parent-Child Non-Matching Autosomal DNA Segments shows the results of a comparison of 2 children’s autosomal DNA against both of their parents.

Perhaps these ordinary cases will help establish a baseline for further analysis.

The examples at the very end of your list are extreme cases of pedigree collapse. Searching for documentation for more common cases (cousins marrying cousins, as you said in your question) would give you more data points. It would be interesting to find a documented case of two sets of twins intermarrying for comparison.

For raw percentages, you could look at the breeding of racehorses, or show dogs and cats, where linebreeding is used, but I don't know how useful it would be to compare cMs from non-human DNA.

See also:

• Kitty Cooper's Blog: Pedigree Collapse or the Family Braid with an example of pedigree collapse from 23andMe results.
• Pedigree Collapse Calculator from Genetic Genealogy Tools