i guess my thinking is that if there was something going on, you'd see it more often... there have been a number of jag x sib pairings, but maybe no other instances of this.
monosomy would not necessarily be noticable in either jag x jag or jag x sib. jag x jag would yield undeveloped leucistics regardless of the occurance, and sib x sib would not yield a leucistic simply because of monosomy... you would most likely just have a dead carpet with some sort of deformation.