'WHY am I brown, daddy?' 10-year-old Michael asks his father. His father cannot explain why.

Mr Keith Williams, 47, and his wife, Catherine, 46, have been going through hell ever since their son was born. The reason?

The couple are white but Michael is dark, a case of in-vitro fertilisation (IVF) gone horribly wrong.

All the names have been changed to avoid identifying the family.

IVF is a process whereby egg cells are fertilised by sperm outside the body. The embryos are then placed in the womb.

The wrong sperm was mistakenly used by the hospital fertility clinic the couple attended in their hope of becoming parents.

What makes the case so bizarre is that their 13-year-old daughter, Susan, conceived three-and-a-half years earlier than their son in the same way with the same sperm donor is light-skinned, The Daily Mail reported.

Now the couple are suing a fertility clinic in Belfast for mental distress. The case, believed to be the first of its kind in the country, is expected to come up for hearing in September.

The case comes about six years after the clinic offered the couple an 'unreserved apology' for the mistake over the labelling.

Why did they take so long to sue the clinic? The couple alleges that the clinic has been stonewalling the case for so long.

The couple may not be the only ones who have suffered due to the mix-up. The clinic, who, according to the letter, also 'achieved pregnancy as a result of using a particular batch of such donor sperm' - presumably with the same unexpected consequences, the report said.

Mr Williams, a businessman, told The Daily Mail: 'People may say I should have told him sooner, but at what age do you explain in a way he can understand? The fact is, we should never have been put in the position of having to explain this.'

Mr Williams said Michael had suffered racial taunts at school.

Said Mrs Williams: 'It's been much harder for Keith because the children are mine biologically, but he is terrified they will reject him when we tell them the truth, because they are already suffering because of the hospital's mistake.'

The Williamses, who live in a small village in Northern Ireland, have been married for more than 20 years. But now they claim their marriage is under tremendous strain.

They tried eight years to conceive. Mr Williams was told he was infertile after he went to the doctor with a swollen testicle - fearing cancer - and further investigation revealed a benign cyst causing a blockage.

Said Mr Williams: 'We did talk about adoption, but the process seemed terribly long. And Catherine wanted to experience pregnancy and bear her own child, so using donor sperm seemed the perfect solution for us.'

That's when they decided to go for IVF.

Mr Williams said he even paid ??pounds;2,000 ($4,800) towards the fertility treatment in cash because he didn't want anybody to find out about it through his bank records.

Daughter looks like them

When Susan was born, they were overjoyed. Recalled Mr Williams: 'I remember Catherine cradling Susan in her arms and saying to me: 'She looks a little like both of us'.'

When Susan was about 2, Mr Williams said, the clinic contacted them to ask them what to do with the remaining fertilised eggs.

Mr Williams claims he was wary, but his wife wanted another baby.

Michael was born naturally in 1998, but the couple couldn't help notice that the baby was darker than normal.

As Michael grew up, he became darker.

When Michael was 3, they received a call from the clinic. The doctors there wanted an urgent meeting 'to discuss a serious matter involving their children'.

They wanted a face-to-face meeting at the earliest.The doctor arrived at the couple's home and delivered the shattering news.

Mr Williams believes the hospital was alerted to the mistake by another recipient of this batch of donor sperm, and then contacted the other families who'd had children as a result.

It is not known exactly how many other families have been affected.

When the Daily Mail contacted the clinic, it said it was unable to comment on ongoing legal cases.

This article was first published in The New Paper