IT HAD been Emily's first day back at school after the long summer break, and her mother's relief was evident. Jenny Herrett, a small blonde woman, curled up on the sofa, leant back and closed her eyes. 'You'll have to forgive me. My view of her is quite negative at the moment,' she said. 'She tends to regress during the holidays.'

From upstairs constant bumps and thuds could be heard as the Herretts' nanny persuaded seven-year-old Emily it was time for bed. Her room is spartan, just a mattress on the floor and some built-in wardrobes that are always locked. It is in Emily's best interest: she has destroyed two beds, attempted to eat the mattresses, and fallen off bedroom furniture.

Mrs Herrett is painfully honest about the demands of coping with a child with Angelman's Syndrome, which combines distinctive physical characteristics with severe mental disability. Her daughter is physically able and very strong, but cannot speak and has limited understanding. She is hyperactive with a short attention span, which tests the patience of her carers to the limit.

'She's like a very naughty child, but there is no deterrent to her naughtiness. You can't threaten her with anything,' her mother says.

Angelman's Syndrome, caused by a genetic abnormality, is increasingly recognised as a cause of severe mental retardation in children. In the past five years, rapid progress has been made in understanding the condition, and the genetic counselling offered to families has improved dramatically. The condition has also excited the attention of scientists, because it provides an insight into a genetic phenomenon known as genomic imprinting.

This was discovered in mice in 1985, when scientists noted that some genes on a chromosome are expressed differently, depending on whether the chromosome comes from the father or the mother. Each cell in the human body contains 46 chromosomes. However, eggs and sperm each contain half this number, so at conception an embryo receives 23 chromosomes from its mother and 23 from its father.

One of Mrs Herrett's eggs contained a damaged chromosome 15: it had a tiny portion of genes missing, and Emily inherited it. But had that damaged chromosome 15 - with the same portion of genes missing - come from her father, she would have suffered from another condition known as Prader Willi Syndrome, that has different symptoms.

'We know now that there are several human conditions which show genomic imprinting, but Angelman's presents us with the best model yet to study this new mechanism of inheritance,' says Jill Clayton-Smith, a clinical geneticist at St Mary's Hospital in Manchester.

Instead of carrying identical information, it is now clear that a particular stretch of the mother's chromosome 15 carries different genetic information to that on the same portion of the father's chromosome 15. In a healthy person, both these chromosomes are present with the full complement of genes.

'Nobody knows why it happens,' says Dr Clayton-Smith. 'It may be that nature did not have enough room to repeat information. Genomic imprinting may be a means of 'saving space'.'

Angelman's Syndrome was first described in 1965 by Harry Angelman, a British paediatrician. Until 1987, when the deletion on chromosome 15 was identified, only 20 to 30 reports of it had been made. Now more than 200 children in the UK are known to be suffering from it, and Dr Clayton-Smith says many more, including some adults, may not yet have been diagnosed.

Emily has the pointed chin, wide smile and jerky limb movements that have led to Angelman's being tagged the 'Happy Puppet' syndrome. However, unlike the majority of sufferers, she does not have epilepsy and her hair is bright red, not the pale, straw-colour that is a common symptom. Other characteristics include a tendency to flap the arms and walk with a wide, stiff-legged gait. None of the children can speak, and all show a distinctive abnormal brain-wave pattern.

At birth, Angelman's children appear to be normal. It is only as they grow, and developmental milestones are delayed, that problems arise. Even so, many doctors will miss the diagnosis.

Mrs Herrett's doctor told her that Emily was 'slow', and the extreme floppiness of her limbs was nothing to worry about. When she was a year old, she saw a paediatric physiotherapist, who suggested that the Herretts take Emily to a paediatrician at Guy's Hospital in London. Two consultations there failed to produce a diagnosis.

'In a way, I hoped that we would never get a diagnosis; that it would be all right in the end, and she was just slow,' Jenny Herrett says.

However, when Emily was two, the Herretts sought a second opinion at Great Ormond Street Hospital, where, in August 1987, she was immediately diagnosed as an Angelman's child. Mrs Herrett had mixed feelings. 'There was relief that we knew what was wrong at last, but the biggest blow was when we were told that there was a one in four chance that we would have another affected child.'

A year later, the Herretts were told that the risk of having another affected child was only one in ten. 'In the end, we just crossed our fingers and took a chance,' she says. A second daughter, Lorna, was born in May 1989, and is perfectly normal.

Research has begun into which genes are missing on chromosome 15. However, the focus of British studies, part- funded by the charity Action Research, at the Institute of Child Health in London, is geared to improvement in genetic counselling. Sue Malcolm, one of the researchers there, says: 'We are trying to find the genes that occur in that particular area, and how they may affect brain development in the long term. But what we really want to be able to do is tell the parents of an affected child whether it will happen again.'

The Herretts, who want to have another child, have already benefited from this research. Both the risk factors they were given - one in four, then one in ten - have been proved wrong by the latest research.

In three-quarters of cases, Angelman's is due to a deletion in the mother's chromosome 15, and scientists now know that the risk of such a family having another affected child is very low (less than 2 per cent). In addition, a prenatal diagnostic test is available for these families, which, in most cases, shows whether an embryo is affected.

In the remaining cases, Angelman's may be due to a child inheriting two chromosome 15s from its father. This has the same effect as a deletion in the mother's chromosome 15, and is known as 'uniparental disomy'. But the syndrome can also arise because of a genetic mutation on chromosome 15.

If uniparental disomy is responsible, the risk of another affected child is again very low. But if a mutation is the cause, Dr Clayton-Smith says, it is very high indeed. 'We can't put a figure on it, and that is a real problem when counselling. There are five families like that in this country.' No prenatal test has been devised to identify a mutation, but Dr Clayton-Smith predicts that one will be available within a couple of years.

As well as genetic counselling, parents want to know just how their child will develop, she says. 'They want to know what their two-year-old will be like at 22.'

The Herretts have many worries about the future, and are realistic in their hopes for Emily. At present she is doing well at the special school she attends near Greenwich. Her parents are keen to try any therapy that they think may help her. At one time, a team of 25 helpers took her through a daily four- hour programme of 'patterning', designed to encourage dormant brain cells to take over from damaged ones.

Earlier this year, they took her to Miami to try 'dolphin therapy' which, an American doctor claims, can help children with speech problems. After a successful attempt at speech, the child is rewarded by being allowed to touch or swim with the dolphin. But her progress is slow, and Mrs Herrett fears that the family may be 'split' as Lorna grows older and develops demands and needs that differ from Emily's.

In the long term, a special home or community is the most likely prospect. 'We don't envisage looking after her all our lives,' Mrs Herrett says. 'That wouldn't be good for anyone, especially not for Emily.' But until then, the Herretts will battle on.

The Angelman's Syndrome Support Group is run by Sheila Woolven, 15 Place Crescent, Waterlooville, Hampshire

PO7 5UR.

(Photograph omitted)