Boston bomber Tamerlan Tsarnaev tricked girlfriend into thinking he'd given her AIDS

Boston bomber Tamerlan Tsarnaev fought constantly with his American girlfriend and once tricked her into thinking he'd given her HIV during sex, a court has heard.

A former roommate of Katherine Tsarnaeva, a Boston resident who fell for Tsarnaev, converted to Islam then married him, described their turbulent relationship to a jury in the city today.

Amanda Ransom, 25, was called to the stand in the ongoing trial to decide whether to execute Tamerlan's brother, Dzhokhar Tsarnaev.

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Sick joke: Tamelran Tsarnaev, pictured, tricked his then-girlfriend Katherine Tsarnaeva into believing she could have caught HIV from him as a joke, her former roommate revealed in court

Convert: Katherine Tsarnaeva, maiden name Russell, converted to Islam after meeting Tamerlan Tsarnaev, whom she later married

Sick joke: Tamelran Tsarnaev, left, tricked his then-girlfriend Katherine Tsarnaeva, right, into believing she could have caught HIV from him as a joke, her former roommate revealed in court

Ransom, who was Tsarnaeva's roommate in her freshman year at Suffolk University, told how Tamerlan reduced his then-girlfriend to tears by making her think she could have caught AIDS from him.

She said she overheard Tsarnaeva, whose maiden name is Russell after the two had sex one night. Ransom said: 'At one point I heard him laughing really hard, and she was crying.'

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It emerged he had just told her that he had AIDS, raising the prospect he had passed her the HIV virus, from which AIDS develops.

Before: Katherine Tsarnaeva, maiden name Russell, is pictured above in a high school yearbook photo before meeting Tsarnaev

After his girlfriend burst into tears, Tsarnaev revealed that he did not actually have the disease and said he meant it as a joke, she said.

She also detailed a fight in 2009 the two had in the home she shared with Tsarnaeva and another woman.

She said she heard items of furniture being thrown at around 2am accompanied by shouting, but was turned away by Tsarnaeva when she went to help.

Ransom said she barricaded herself into her room before later fleeing the house with her other roommate in their pajamas.

She said Tamerlan threatened her that night, saying: 'If I ever see you again...'

Earlier in the trial jurors were told how Tsarnaeva converted to Islam after a difficult beginning to her relationship with Tsarnaev, who cheated on her.

But after becoming pregnant with his child, the two married. Tsarnaeva continued following Islam after it emerged her husband had been involved in the bombings.

Defense lawyers for Dzhokhar Tsarnaev have argued that Tamerlan had a strong influence on his younger brother, and manipulated him into helping with the bombings.

Prior witnesses called by the defense have testified that Dzhokhar Tsarnaev was very unlike his aggressive brother, growing from a happy child into an easygoing young man who liked Domino's pizza and rap music.

His attorneys have argued that this makes him less deserving of the death penalty, and have said he should instead be given life without parole in a high-security federal prison.

On trial: The anecdote was recounted in the trial of second Boston bomber Dhozkhar Tsarnaev, pictured, who is facing the death penalty

A Princeton history professor was also called to the stands Tuesday, who described the background to the history of Chechnya, the Tsarnaev family's ancestral home.

The nation has been riven by jihad in recent decades, Professor Michael Reynolds pointed out - though Dzhokhar and Tamerlan spent relatively little time living there.

He also said that the father and eldest son in Chechnyan families have clear roles as the central decision-maker and that Dzhokhar would likely have felt a sense of 'deference' and 'loyalty' to Tamerlan.

Prosecution lawyers have objected to the line of questioning at times, interjecting to ask witnesses examined on Tamerlan whether they had ever met Dzhokhar.

When asked the question Tuesday, Ransom said she had not met him.

Witnesses were also called Tuesday who spoke about the bombers' father, Anzor Tsarnaev.

A doctor told the court who Anzor, who lived in Boston with the rest of the family, suffered from seizures, post-traumatic stress disorder - and believed he was being tailed by Russian spies.

Alexander Niss, a psychiatrist who treated him from 2003 through 2005, told the jury 'he was a very sick guy'. He said he was so debilitated he could neither drive nor work.

Lawyers argue that their father's illness left a power vacuum which Tamerlan was able to fill.

Tsarnaev, 21, was found guilty last month of killing three people and injuring 264 others with a pair of homemade pressure-cooker bombs at the race's crowded finish line on April 15, 2013, as well as fatally shooting a police officer three days later as Tsarnaev and his older brother prepared to flee Boston.

Power vacuum? A doctor told how Anzor Tsarnaev, the bombers' father, was 'a very sick guy', which defense lawyer argue would have given Tamerlan more sway in the family

Defense lawyers, who at the trial's opening in March conceded that Dzhokhar Tsarnaev had committed all the crimes of which he was accused, contend that 26-year-old Tamerlan was the driving force behind the bombing, with his younger brother coming along out of a sense of sibling loyalty.

Tamerlan died early on April 19, 2013, following a gunfight with police.

On Monday, the jury heard from Tsarnaev's Russian relatives, who testified that he had been a sweet-tempered child and that they had been distressed when his mother and Tamerlan began to espouse radical Islamic views after moving to the United States.

Tsarnaev started crying during testimony from one of his aunts - the first obvious show of emotion he has made throughout the proceedings, which have included heart-rending testimony from the bombing's victims.

Plots: Defense lawyers for Dzhokhar Tsarnaev (left with his older brother) are trying to convince jurors that Tamerlan was the driving force behind the bombings

Federal prosecutors earlier presented evidence showing that Tsarnaev's computers contained radical jihadist literature including copies of al Qaeda's Inspire magazine and noted that he left a note suggesting the attack was an act of retribution for U.S. military campaigns in Muslim-dominated countries.

The judge in the case has said the defense will take about two weeks to present its case in hopes of sparing Tsarnaev's life. The defense is now in its second week of calling witnesses.

Martin Richard, 8, Chinese exchange student Lu Lingzi, 23, and restaurant manager Krystle Campbell, 29, died in the bombing. The Tsarnaev brothers shot dead Massachusetts Institute of Technology police officer Sean Collier three days later.