One in 2000 people may have two copies of a chromosome from one parent

And none from the other. And they may show no abnormalities:

To date, most cases of uniparental disomy—having two copies of a chromosome from either mom or dad, rather than one from each—have been identified in the context of disease, but a new study from researchers at the direct-to-consumer genetics company 23andMe finds that this phenomenon is more common than previously realized and exists in many healthy individuals. The study, published yesterday (October 10) in the American Journal of Human Genetics, suggests that uniparental disomy (UPD) occurs in about one in every 2,000 people.

Jef Akst, “Unbalanced Chromosomal Inheritance More Common than Thought” at The Scientist

Apparently, it’s twice as common as once thought. It’s amazing how much the human genome has learned in recent years.