Allele frequency refers to how often a specific version of a gene, known as an allele, appears in a population. It is usually expressed as a proportion or percentage. For example, if a particular allele occurs 30 times in a population of 100 individuals, its frequency would be 30%. This measurement helps scientists understand genetic diversity and how traits may be passed on through generations. Changes in allele frequency over time can indicate evolutionary processes, such as natural selection or genetic drift. By studying these frequencies, researchers can gain insights into how populations adapt to their environments and how certain traits become more or less common.