[coconutt2000]

which neither increases nor reduces a human’s chance of survival.

Umm... Chances of survival have nothing to do with passing on genes. Chances of reproduction however... That's the ticket.

[CholeraJoe]

In some recessive traits, being heterozygous (carrier state) infers resistance to certain infectious diseases. Examples include sickle cell anemia (malaria), cystic fibrosis (tuberculosis) and Tay-Sachs (typhoid).