Mobley, 25 and with a long and violent criminal record, had admitted shooting a pizza store manager in the back of the head during a failed robbery four years ago. But his lawyers argued he should be spared the death penalty because of a defect in his genetic make-up.
Mobley's family tree is littered with incidents of criminal and violent behaviour. His mitigation focused on a direct chain of antisocial behaviour that could be traced from his great- grandfather.
His lawyers tried to adduce expert evidence to show that a gene mutation had been passed along this line and was ultimately responsible for the disastrous events on 17 February 1991 at the pizza parlour in County Hall, Georgia.
As long ago as 1969, genetic evidence was first admitted in a New York court. Lawyers then put forward a genetic-defect defence concerning the XYY chromosome syndrome. They argued that the extra Y chromosome indicated greater "maleness" or aggression. However, it failed to gain widespread judicial acceptance.
Mobley's lawyers introduced evidence of a recent Dutch study, which associated this sort of family aggression with chemical imbalance caused by a mutating gene. Nevertheless, the Georgia Supreme Court held this evidence to be inadmissible on the basis that the theory of "genetic connection is not at a level of scientific acceptance that would justify its admission".
The court's reluctance flies in the face of American legal opinion and a usually science-friendly US judiciary. In a recent survey carried out by a US law journal, almost every attorney interviewed agreed that it was only a question of time before genetic mitigation was allowed.
Dr Deborah Denno, associate professor of law at Fordham University School of Law in New York, visited London in February to present a paper on the legal implications of this type of genetic research. She believes that in five years' time such evidence will be admitted into US courts. "Given that so many people who commit homicides also have histories of families with relatives who are also incarcerated, I think it's just a matter of time before somebody tries it again," she says.
In Britain, genetic research has established DNA profiling as a weapon in the prosecution's armoury. On 10 April, the world's first DNA database for criminal records was opened at the Home Office's forensic science laboratory in Birmingham. If genes can be used to convict, it could be argued, why can't they be used to acquit?
British defence lawyers are sceptical about the usefulness of the so- called "criminal genes". Roger Ede, secretary of the Law Society's criminal law committee, believes that to defend a client by saying he or she is "born to be bad" is a double-edged sword. "There's a fundamental problem with this. To say your client is inherently bad means you can't say they're going to get better.
"Mitigation centres on the person feeling contrite and having learnt a lesson from having been taken to court. You can't plead that at the same time as pleading that they are inherently bad and beyond salvation," he says.
Steven Kay, secretary of the Criminal Bar Association, says: "I don't think much of the idea that because the rest of the family has been at it the courts should view the defendant before them sympathetically."
He believes that "social cohesion" requires people to be responsible for their own actions. "It will soon get to the stage where you could find an excuse for anyone's behaviour for anything."
Dr Denno argues that the value of genetic mitigation is that it can show how a guilty offender is not completely responsible for his or her actions and therefore should not be punished as severely as somebody who knows better. She says it has been used successfully to explain how wrongdoers do not always have complete control over their behaviour.
An American attorney who suffered from alcoholism and swindled his clients managed to persuade the legal authorities not to disbar him. "In that case," says Dr Denno, "the genetic link to alcoholism was a useful argument."
In another case, a woman with Huntingdon's disease who tried to kill her two children also amassed genetic evidence to support her argument that she deserved lenient treatment.
"A jury might feel some sympathy for somebody who has been a rather peaceful person and then suddenly tries to murder her children if there is genetic evidence for her violent behaviour," Dr Denno says.
Many genetic scientists doubt that there are specific genes that make people commit crime. The accepted view is that genes and environmental factors have a combined influence on behaviour.
Dr Denno is careful to distinguish between genetic mitigation and genetic defences. "I think it's going to be a long time before it [evidence of genetic defects] is used as a defence. But the defence would be that the person did not have the intent to commit the crime because of a lack of control in their behaviour; because they are genetically predisposed to commit violence."
Roger Ede believes lawyers who rely on this defence may end up helping to have their client more severely punished: "Offenders who can't be helped are simply taken out of circulation."
He says this is "probably the last thing you want if you are appealing to the court that there are things that can be done, other than sending your client to prison, which are going to help your client be a better citizen." However he accepts that if there were a treatment for this type of genetic condition, the argument would have more force.
In American states such as Georgia, where convicts face years on death row, lawyers only need to convince the court of an alternative to capital punishment. In those circumstances, genetic defences don't seem quite so fraught with risk. Dr Denno says: "All defences run some risk or another. What do you have to lose when you are on death row?"
In the Mobley case, Stephen Mobley has changed his lawyers and withdrawn his genetic defence in his next appeal against his death sentence. His father, a multi-millionaire who did not suffer from the genetic defect attributed to his son, was "embarrassed" by publicity given to the case and the details of the family history revealed in the press.
"Everybody has a family skeleton in their closet," says Dr Denno, pointing out that if it's a life-or-death matter, you do whatever you can to get your client off death row.Reuse content