Nicole's spirit, her place in our family, is the driving force behind

In the front garden of their home, the Masterton family have planted a cherry tree in memory of the little girl they lost just over a year ago.

It is surrounded by busy lizzies and marigolds and at its foot is a plastercast bunny rabbit and three-year-old Nicole Masterton's pink spade.

Nicole liked to help her mother, Louise, in the garden. She loved flowers and the immaculately kept blossoms around her memorial represent her favourite colours of pink and orange.

But since Nicole died two months after falling into a fire in her back garden in May last year, Louise, who has four sons, says she has lost the special relationship that a mother has with her daughter.

The Mastertons tried for 15 years to have a girl and when Nicole passed away they say the "female dimension" of their family was lost.

Now they hope to use the European Convention on Human Rights, introduced into UK law last week, to win the right to choose the sex of their next child to replace the little girl they lost and redress the gender imbalance which they say is essential to make their family complete.

While the Mastertons announced their legal challenge for the right to decide the sex of their child, it emerged that doctors in America used genetic screening to select a test-tube baby with precisely the right cells to enable him to act as a donor to his seriously ill older sister. When Adam Nash was born on August 29 this year in a hospital in Minneapolis, doctors collected cells from his umbilical cord which were then infused into his elder sister, Molly, who suffers from Fanconi anaemia, which stops cell production in the bone marrow. The condition is universally fatal and without a transplant Molly would certainly have died. Thanks to her genetically selected brother she now has a 90% chance of survival.

But now both the Nashes and the Mastertons have been accused of designing babies to meet the needs of others and their stories have prompted a fierce ethical debate that advances in technology will inevitably be used by parents to choose the physical characteristics and even intelligence of their children.

While most experts concluded that genetic selection in the case of the Nash family was ethically acceptable because it was for the medical benefit of another human being, the Mastertons have been accused of overstepping the mark by wishing to select the gender of their next child for "social reasons".

But the Mastertons deny their daughter would be a "designer baby." They insist they should have the right to do what they believe is best for their family and point to psychological reports which state that another girl in the family would be good for the four boys.

Sitting in the front room of the family home in Monifieth, near Dundee, Louise lifts one of the many photographs of Nicole from the mantelpiece. The 42-year-old explains that when she finally had a daughter she felt complete and is now missing an essential part of herself: "I felt whole and complete right to the core when I got Nicole. I don't feel like that any more."

Nicole's napkin ring and teddy bear, Tuggie, are on display, surrounded by old-fashioned dolls. "These are my angels. One is called Keep Me Safe and the other is called Jesus Loves Me. I got them after Nicole passed away because she was an angel," she said.

Louise recalled the things she and her daughter did together and the things she had planned to do in the years to come: "We used to do the garden together because Nicole loved flowers and I always looked forward to going shopping with her when she got older.

"I used to keep saying I will keep going until I a get a girl. I reassured the boys that I loved them but told them, 'you will not want to come shopping with me for a frock'."

Alan Masterton loves his four sons but points out that he had a different relationship with his daughter. She was more openly affectionate.

"Any father who has a daughter and sons will be able to tell you that a daughter is just different. I would dry Nicole after she had had a bath and she would sit with me and have her supper and she would give me a cuddle," he explained.

But the Mastertons are aware of allegations that they want another girl for their own selfish reasons. They repeatedly say that the incentive is to replace the "female dimension" to make the family complete - but admit that explaining what this means is not easy.

"It was Nicole and the four brothers. They were more protective towards her, they wouldn't have been like that towards the younger brothers. If the boys went to the sweet shop they would all bring her back a little bag of sweets. They wouldn't have done that for their younger brothers. It was because she was a girl. The boys would adore another sister," says Louise.

The couple are also aware of the medical and ethical controversy and the reasons why gender selection should not be allowed. But in a 30-page document submitted to the Human Fertilisation and Embryology Authority (HFEA) they explain why they should be made a special case.