Em primeiro lugar, esse é um cromossomo fictício, para que eles possam fazer o que quiserem no roteiro do filme.
Quanto ao filme, o 24º cromossomo deveria dar aos humanos super poderes, em primeiro lugar, não transformá-los em monstros. E é isso que ela percebeu, que realmente funciona, mas depende da pessoa (não capaz de ser infectado = nem todo mundo vai se transformar em um monstro)
The purpose of the new chromosome was to transform individuals into superhumans, with enhanced strength, agility, and endurance. The Olduvai scientists experimented with the chromosome, injecting it into convicted murderers. However, instead of becoming superhuman, the test subjects transformed into hideous monsters, and the scientists lost control of the facility.
Samantha has come to the conclusion that what the chromosome does to a person depends on whether or not that person has certain tendencies; essentially coming down to whether they are inherently "good" or "evil"; good people become superhuman, as planned, but evil people become mindlessly violent monsters, who instinctively "infect" other evil people with the chromosome and murder good people.