centimorgans between half relations

[Snowstorm]

I have DNA tested at Ancestry, my centimorgans match with my full sibling is exactly as expected. We have a half sibling who has not tested, but our half nephew and half great-niece have both tested and both are measuring at about half the expected centimorgans against both myself and my full sibling according to the cousins charts I have consulted. Does anyone have any explanation as to why this might be?

[Mart 'n' Al]

Are you using something like this:

https://dnapainter.com/tools/sharedcm

Are your findings not even between the upper and lower limits?

Martin

[Snowstorm]

That's the chart I used.

[Mart 'n' Al]

I shall watch this thread as I think I might learn something.  Can you be totally sure of those relationships that you mentioned?

Update 21:38
A sibling should be about 2629.  Are you reading the chart correctly?

Martin

[Snowstorm]

Yes

[Mart 'n' Al]

The half nephew should be about 900.

Martin

[sugarfizzle]

Snowstorm, First of all, a big welcome to Rootschat.

What sort of discrepancies are you talking about?

Looking at your half nephew and half great niece, if they share a great deal less than 500 cM and 125 cM, I don't perhaps think there is an explanation.

I presume, maybe incorrectly, that these two are son/grandaughter of your half sibling. If so, could she be persuaded to test, to broaden the picture?

It might be worth submitting your own results to the project, which I believe is still running, though it has not been updated for a while.

https://docs.google.com/forms/d/e/1FAIpQLSc5a0SIHIeiwLl5Wxn4sLqgnRV-su2klK2W_YzIJc9xq2i4zw/viewform

Sorry not to have been more helpful, but DNA inheritance is sometimes a strange thing. 

Regards Margaret

[sugarfizzle]

I usually refer to the original shared cMs project, though I do use DNAPainter.
Whilst looking at the link which Martin provided, I saw this.

https://dnapainter.com/tools/sharedcmv2

Try adding your own names/numbers, see what it comes up with. It won't alter what you know, but it gives an immediate indication of what the project so far has come up with.

Regards Margaret

Modified: Link to original

https://thegeneticgenealogist.com/2017/08/26/august-2017-update-to-the-shared-cm-project/