A heterozygous advantage is where carrying just one autosomal recessive mutant allele could provide a selective advantage to the individual, by providing protection from a disease, for example. This can explain why alleles which are lethal or deletarious in the homozygous form persist .
For examples see:
- ↑ Evolution.berkeley.edu, (2014). The "bad" gene. [online] Available at: http://evolution.berkeley.edu/evolibrary/article/misconcep_04 [Last accessed on 22 November 2014]