In article , Claudio Jolowicz
writes

Pear and apple are both haploids with 17 chromosomes, probably descending
from an ancient polyploid. The structure of their chromosomes seems to be
very similar, as you said that little modification has occurred in it,
caused by duplications, deletions or inversions, during their evolution
from that polyploid, and (I guess) little enough to cross them.


Conservation of the structure of the chromosomes is an inference (hence
the use of the word 'suggests' in my prior post); it may not be a
correct inference. The inference is perhaps testable by examination of
chromatin banding patterns. Also by sequencing of genomes, but this
would be a very expensive test to perform, and can't be expected for
some decades.
--
Stewart Robert Hinsley