Angelman syndrome - Colin Farrell's four-year-old son has it

This is one of two syndromes (Angelman and Prader-Willi) which have different presentations depending on which parent - the father or the mother had the defective genes. That means, we can know whether Mr Colin Farell had the defective genes or the mother of the boy.

Angelman syndrome comes from defective maternal chromosome 15, so, it was from the mother. And, we can also conclude that Colin Farell's son's children will not have Angelman syndrome but rather has a high chance (25%) of getting Prader Willi syndrome.

Angelman syndrome (AS) is a rare neuro-genetic disorder named after a British pediatrician, Dr. Harry Angelman, who first described the syndrome in 1965. AS is characterised by intellectual and developmental delay, speech impediment, sleep disturbance, unstable jerky gait, seizures, hand flapping movements, frequent laughter/smiling and usually a happy demeanour. AS is a classic example of genetic imprinting caused by deletion or inactivation of critical genes on the maternally inherited chromosome 15.

The sister syndrome is called Prader-Willi syndrome, and is caused by loss of paternal genes.

An older, alternative term for AS, happy puppet syndrome, is generally considered pejorative and stigmatizing so it is no longer used, though it remains useful as a diagnostic heuristic.

Colin Farrell said Tuesday that his 4-year-old son James, who has a rare neuro-genetic disorder called Angelman Syndrome, is making progress – and that father and son will be together at the holidays.

The actor, 31, told PEOPLE at a New York screening of his film Cassandra's Dream that James's progress is "good" and stressed that the Angelman is a condition, "not a disease."

With the holidays coming up, Farrell says he's looking forward to some bonding time with James. "Yeah, absolutely," he said. "All the family will be together. What are we going to do? Nothing. The usual – watch movies, eat food ... take it easy."

Farrell shares custody of James with ex-girlfriend Kim Bordenave, the boy's mother.

One sign that Colin Farrell may be mellowing out: He hit Las Vegas this weekend, but avoided the city's famed nightlife.

Instead, the Irish actor, 31, spent his time with his girlfriend of six months, Muireann McDonnell, reportedly a 22-year-old student from Dublin.

Farrell and McDonnell were spotted smooching in their hotel pool on Friday, and dined that night at Morton's steakhouse. The couple also caught the Cirque du Soleil shows O and Love, where they sipped water, not alcohol.