A British couple has had a so-called "designer baby" who they hope will be able to help their existing sick child.

BBC News Online looks at why they have gone ahead with the controversial procedure and how their situation differs from that of other families wanting to "design" a baby.

What has happened?

Michelle and Jayson Whitaker have a four-year-old son Charlie who has a rare condition called Diamond-Blackfan anaemia.

It means he needs painful daily injections to keep him alive.

There is only one cure for the condition - a transplant of "stem cells" from a matching donor to kick-start blood cell production.

Neither Michelle, Jayson or their 23-month-old daughter Emily are a match.

But their new baby Jamie was "designed" to be a genetic match.

Stem cells can now be taken from Jamie's umbilical cord blood to treat Charlie.

Are the Whitakers the first UK couple to have a "designer" baby?

No. In 2001, a British woman gave birth to a baby genetically designed to provide tissue to help her four-year-old son who had leukaemia.

How can the Whitakers be sure their new baby is a genetic match for Charlie?

Embryos were tested to see if they were a genetic match with Charlie.

Two embryos, created using IVF, were selected and implanted in Michelle's womb.

The second embryo did not survive.

Any child conceived naturally by Michelle and Jayson had a one in four chance of being a match.

The chances using the tissue-typing technique increase to around 98%.

Were the family allowed to have the treatment in the UK?

No. The family from Bicester were refused permission to go ahead with the treatment in the UK.

So they travelled to the Reproductive Genetics Institute in Chicago.

Jamie was born in the UK on Monday, at Sheffield's Royal Hallamshire Hospital.

Is this case not similar to that of the Hashmis, who have been given permission to have treatment in the UK?

No, there is a significant difference between the two cases.

The Human Fertility and Embryology Act 1990 allows genetic selection of embryos if it is the new child's interests; if there is any risk that they will be born with a specific disease.

Raj and Shahana Hashmi wanted to have a baby who could help their four-year-old son Zain who has the rare blood disorder thalassaemia.

They were allowed to use the tissue typing technique to select an embryo because the condition is genetic, and there is a strong chance any baby conceived naturally would have the condition.

But there is no genetic test for Diamond Blackfan anaemia, and there was only around a one in 50 chance of the Whitakers having another baby with the condition.