OK, ancestry says I share 3,421 cM with my Dad across 81 segments, while My Heritage says 3,534 cM across 22 segments. So in this instance, MyHeritage is counting each full chromosome as a segment, because all the DNA on 1 of each pair of my chromosomes comes from one or other of Dad's corresponding chromosomes. It doesn't split the chromosomes into segments based on which of his 2 chromosomes any particular bit of DNA came from. As for ancestry, I know they filter out some of the common Jewish DNA, so I don't know whether that is why they say 81 segments, or if they do actually only count as a segment if it is all on the same one of his chromosomes. I can't find an example of a typical parent / child comparison on Ancestry at the moment to see whether they usually say 22 segments or not.
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