A supposedly rare genetic quirk might be more common than we think, according to new research out Thursday. The study, based largely on 23andMe data, suggests that one in every 2,000 people are born with two copies of a gene from only a single parent, often with no serious health consequences.

Ordinarily, a person’s egg or sperm cells have one set of the genes that make up their chromosomes (other cells in our body have two sets). When a sperm fertilizes an egg, the resulting fertilized zygote will then have two sets of 23 chromosomes, one from each parent, making 46 chromosomes in total. If all goes well, the zygote multiplies and divides until it becomes a person, one with an even allocation of gene copies from both parents.