What is gene frequency? What are the circumstances under which it is likely to take place?

The frequency of occurrence of an allele in relation to that of other alleles of the same gene in a population is called gene frequency.

 

Copies of alleles can be lost because they never make it into gametes.

If the allele copy makes into a sperm, that sperm isn't the one that fertilizes an egg. Maybe the organism that carries copies of the allele in its gametes fails to find a mate, or is killed before reproducing.

These kinds of events can influence the frequency of that alelle in a population.