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Rare genetic quirk twice as common as thought

14 October 2019
Appeared in BioNews 1019

Inheriting two copies of a chromosome from a single parent is more common than previously thought, according to a new study.

The phenomenon, called uniparental disomy (UPD), occurs due to an error in meiosis, the process that forms eggs and sperm. Because of the error, the affected individual inherits both copies of a chromosome from one parent only. Documented cases of UPD are rare, with only around 3300 recorded in total worldwide.

Using a dataset of 4,400,363 participants from 23andMe and another of 431,094 UK BioBank participants, US researchers identified 675 instances of UPD. To determine how often UPD occurs in the general population, they also looked at the rate of UPD among trios of parents and children, finding 105 cases within 214,915 trios in the 23andMe dataset. This amounts to roughly one in every 2000 births, compared with a previous estimate of one in every 3500.

'So that's about twice as common as was previously thought,' lead author Dr Priyanka Nakka told Gizmodo.

Existing research has linked UPD to developmental disabilities and a greater risk of cancer. However, while the study found some association between the UPD of chromosome 22 and the risk of autism, it identified no significant associations with deleterious traits in the 23andMe database.

'Our work challenges the typical view that errors in recombination are strongly deleterious, showing that even in extreme cases where individuals are homozygous for an entire chromosome, those individuals can be, to the best of our knowledge, phenotypically normal and healthy,' wrote senior study author Dr Fah Sathirapongsasuti at 23andMe, and colleagues, in their paper.

'We found that a little surprising,' said Dr Nakka, who did the study while at Brown University in Providence, Rhode Island, and 23andMe. 'Because in the past, UPD is always been written about as this genetic phenomenon that can cause imprinting disorders or unmask deleterious mutations.'

To identify UPD in such large datasets, the researchers developed a machine-learning classifier that assessed the degree of homozygosity (possessing two identical alleles of at least one gene).

The classifier struggled, however, with data from populations with higher-than-average levels of homozygosity, such as Ashkenazi Jewish, Middle Eastern, and South Asian populations. This was acknowledged by Dr Nakka and her co-authors, who suggested that individuals who have so far signed up for 23andMe aren't completely representative of the general population.

Nevertheless, the study has provided a clearer picture of UPD. 'I was really excited to see this paper,' Dr Wendy Robinson, a medical geneticist at the University of British Columbia in Vancouver, Canada, who was not involved in the study, told The Atlantic. She had suspected that UPD occurs in healthy people more often than reported.

The study was published in the American Journal of Human Genetics.

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