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Two young girls in Vietnam are living proof that there is a lot more to sex than genetics, after being found to have an extremely rare sexual development condition.

Thanks to their doctors in Hanoi and researchers at Hudson Institute of Medical Research, the siblings have been identified as XY females, and the exact source of their genetic difference has been identified for the first time.

The findings are detailed in The Journal of Clinical Endocrinology & Metabolism.

An incredibly detailed analysis of the siblings' genetic makeup showed that a particular combination of variations from each parent gave them an external female appearance and low testosterone levels despite having XY chromosomes.

The analysis was conducted by Ph.D. student Shirin Moradi Fard under the supervision of DSD specialist Professor Vincent Harley.

Working with clinician-researcher Dr. Uyen Le, they studied the children's genomes to a level of detail that has rarely been attempted previously, resulting in just the fourth confirmed case of this specific DSD worldwide.

"We describe the condition as compound heterozygote, the first example in this condition," said Harley. "That means a single genetic variation has been passed from each parent. These variations have no adverse effect individually, but when combined, they cause this DSD."

Moradi Fard says a mind-boggling level of genetic detail was examined to reach a confirmed diagnosis.

"Initial genetic tests looking at the coding regions of the patient DNA were inconclusive, so their doctors asked Le and the team to consider genome sequencing ," she said.

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