In a statement to police, Zysk said that he and Gonzalez had used heroin earlier that day. He said he was tired and could not get to sleep and grew increasingly frustrated because Jason would not stop crying.
Zysk told detectives he pressed down on the toddler's chest three or four times and then "thumped" the child's side with his fist three or four times.
According to the city Medical Examiner's Office, the punches pushed the boy's liver against his spine, rupturing the organ and causing him to bleed to death.
Assistant District Attorney John O'Neill has asked the jury to find Zysk guilty of third-degree murder.
Defense attorney Evan T.L. Hughes has argued that Zysk lost control and "made a horrible mistake" but should be found guilty of a felony count of involuntary manslaughter.
O'Neill finished the prosecution case Wednesday afternoon and Patricia Zysk was the first defense witness.
Hughes said Joseph Zysk will testify in his defense when the trial resumes Thursday.
Questioned by Hughes, Patricia Zysk said she was "shocked, in disbelief" when homicide detectives arrived at the house the next day and took her son in for questioning and then charged him in Jason's death.
"He had never been in trouble before," she said.
She said her son was always good around children, including his own 3-year-old daughter and Jason.
When Hughes asked Zysk about the impact the case has had on her and her family, she dissolved into tears.
"It's been horrendous, pretty horrendous," Zysk sobbed. "However, there's another grandmother in the room who lost her grandchild and I didn't lose my grandchild and I can't imagine what she feels. I am so sorry."
Questioned by O'Neill, Zysk, a therapist, maintained that she was not trying to minimize what happened or help her son avoid punishment.
"Yes, I'm his mom and I love him, but I don't . . ..," Zysk sighed and paused. "I also believe there are consequences. There's the mother in me and there's that other part of me."