Scientists have created the first genetically modified monkeys that can pass their new genetic attributes to their offspring, an advance designed to give researchers new tools for studying human disease but one that raises a host of thorny ethical questions.

In this case, the Japanese researchers simply added genes that caused the animals to glow green under a fluorescent light and beget offspring with the same spooky ability in order to test a technique they hope to use to produce animals with Parkinson’s, Huntington’s and other diseases.

The work, described in today’s issue of the journal Nature, was hailed by some medical researchers as a long-sought milestone that could lead to crucial insights into a host of ailments and provide invaluable ways to test new treatments.

But the research was condemned by animal rights proponents, who said it paves the way for producing colonies of primates conceived expressly to suffer a plethora of cruel illnesses and undergo potentially painful and dangerous medical experiments.

Because the work marks the first time a species so closely related to humans has been genetically altered in this way, some also worried the same techniques would be used on chimps or other primates even closer to humans or to try to endow people with desirable genetic traits.