Will Arnett -- Good Genes or Good Docs?

Will Arnett's good looks are arresting.

Here's a 29-year-old version of the "Arrested Development" star at a party back in 1999 (left) and 19 years later ... the father of two -- who turned 48 years old earlier this week -- at a convention in Las Vegas just last month (right).

Gob well done.

The question is ...