Fact 1: Amutation in the sex cells of a parent can cause a new trait to appear in the parent 's offspring.

Fact 2: Alleles arise by mutation.

Given the two facts above, answer the question "What can cause new alleles to appear?" with the following options: 
- skin cells
 - coded genes
 - sperm and egg fuse
 - hair cells
 - the parent's sex cells
 - lung cells
 - male gametes
 - reproduction


 

     the parent's sex cells    