Evolutionary advantages of sexual reproduction.

 Parasitism is when an organism is living off of another organism. In order for a parasite to infect the host it has to be able to bind to it, like a lock and a key. When the parasite invents a new key, the host must change the lock or keep getting infected.

The red queen hypothesis states that sexual reproduction persists because it enables the host species to evolve new genetic defenses against parasites that attempt to live off them. So if the key parasites fits the lock of host generation 1, then the host knows that it does not want that lock anymore and generation 2 will have a new lock that the key parasite does not fit. 

If the host carries 2 different forms of the lock gene then that is called heterozygosity. Heterozygosity can have advantage by providing some sort of benefit, like with sickle cell anemia, if you have both forms of the gene (are a carrier of the disease) then you have protection against malaria.