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Hi Gadget,

Unfortunately Ancestry ProTools now costs £7.99 monthly.

pughcd

Thanks for your thoughts Biggles50. I may have the  shared cM data from the closest cousin. At the end of the day I am trying to decide on a full or half cousin relationship and sometimes the data is not conclusive either way!

pughcd

Hi everyone,

I have three DNA matches on Ancestry who all share a great grandparent couple. They are all from the same generation and two of them are brother & sister.

I believe I am related to their great grandfather. Indeed he attended my grandmother's christening in 1905 and acted as her sponsor. Now there is a possibility that the gentleman and my great grandmother might be half siblings rather than siblings so I wanted to test the theory using "The Shared cM Project 4.0 v4".

I have put my shared cM results below;

1) cousin X, 149cM, 3C = 17% probability, half 3C = 6% probability,

2) cousin Y (brother), 60cM, 3C = 22% probability, half 3C = 30% probability,

3) cousin Z (sister), 63cM, 3C =23% probability, half 3C = 32% probability.

So taking the brother (Y) and sister (Z) together one might assume we are half 3C  but the addition of the other cousin (X) gives a more balanced result. As these cousins are all from the same family, sharing a great grandparent, I am taking the view that the low result of 6% for cousin X with whom I get  the closest result (149cM) is telling me that 3C is the best overall description for all of the three cousins. This would indicate that my great grandmother and the great grandfather are full siblings and that my cousin matches are full 3C.

Does this sound reasonable?
Thanks in advance.
pughcd

@JMB1943

The widely accepted rule is that we all inherit 50% of our DNA from each parent.

However, some people argue that whilst the autosomes are generally the same size, the X and Y chromosomes are not equal in size so theoretically  males (XY) inherit slightly less DNA than females (XX). The Y-chromosome is one third of the size of the X-chromosome and contains about  55 genes while the X-chromosome contains about 900 genes.
https://www.genome.gov/about-genomics/fact-sheets/Chromosomes-Fact-Sheet


When you take an autosomal DNA test like Ancestry DNA , 23andMe, Ft-DNA you may be getting some X and Y chromosome analysis but as far as I am aware only FT-DNA reports the X-chromosome data in the results.
https://isogg.org/wiki/X-chromosome_testing and https://isogg.org/wiki/Autosomal_DNA_testing_comparison_chart

It is never 60:40, 70:30 or 80:20.

Ancestry cousin predictions are fairly general but improved data at the DNA Painter website might be more helpful. If you check the Shared cM Project at DNA Painter and enter 389cM it gives a 73% probability for great-great aunt/uncle, half great aunt/uncle, half great niece/nephew, great-great niece/nephew, half 1C or 1CIR. Check the website for other possibilities.

pughcd