11-month old Dallas-born twins Justin and Jordan have different fathers, a phenomenon known as heteropaternal superfecundation that is so rare there are only a handful of documented cases in the world.

Their parents Mia Washington and her fiancée James Harrison went public with their news last week when they contacted FOX4 to tell their story.

Admitting she was having an affair with another man at the time the twins were conceived, Washington said she was shocked that it had happened to her:

"I have twins, but they're by different fathers," she told the TV news company.

Washington and Harrison noticed that the twins had different facial features and decided to have a paternity test. They went to Dallas DNA Lab Clear Diagnostics who said they had never seen a result like this before and that there was a 99.999 per cent chance that Justin and Jordan were fathered by different men.

The highly probable result is that Jordan is Harrison's biological son and Justin's biological father is the man Washington had an affair with.

Lab Clear Diagnostics' president Genny Thibodeaux described the news as "very crazy", and "most people don't believe it can happen".

Dr Chris Dreiling, from the Paediatric Association of Dallas, who has not met the family, told Fox News that a woman can release more than one egg during ovulation, and if she has intercourse with more than one partner at around that time, then sperm from different partners can fertilize each egg:

"Because sperm cells take a while to travel and eggs take a while to travel there can be an overlap," said Dreiling, explaining that it was a very rare event and likely to be "the only time that we will ever see this occur in the city of Dallas".

Harrison said he will bring up the two boys as his own. He said he has forgiven his fiancée and promised to stay with her.

He said they are taking it day by day, "it's going to take time to build that trust like we had", he added.

Washington said she felt wary at first, thinking he would try but then give up and leave. But she said that has not happened.

"James is a good man; he's a great father and genuinely loves both of the twins," said Washington, adding that as far as she was concerned he was the father of both boys because "he's the one there every morning when they get up and every night when they go to sleep".

Washington said she regrets her mistake and wants other people to know that this can happen.

"Don't put yourself in my shoes, because it can hurt and it does hurt, but you still have to go on with life". She had this advice for other women:

"Be careful about starting an affair - look what happened to me. Think hard about the consequences first, because the most bizarre things can happen when you least expect it!"

Washington said she will tell her sons about their different DNA when they are old enough to understand. She has no plans to tell the other father, although she did say "if when he is older Justin wants to meet his real dad then that's his decision".

Although rare in humans, heteropaternal superfecundation is more common in other animals such as cats and dogs, a fact that is well known to professional breeders. Some kennel clubs for instance allow the registration of litters to more than one male dog or "sire", a phenomenon called "multi-siring", and you can purchase DNA test kits from them for that purpose.

Sources: myFox DALLAS/FORTWORTH, Daily Mail, American Kennel Club.

Written by: Catharine Paddock, PhD