What determines how often a phenotype occurs in a population?

An allele being dominant will determine frequency of occurrence of any specific phenotype.  

Explanation:

The occurrence of any phenotype is dependent on the allele. If the allele is dominant, there are high chances of occurrence of a particular phenotype again and again. However, if the allele is recessive, the chances of occurrence of any phenotype associated with recessive allele are low as recessive traits are expressed only when they are in pairs.  

For example –  

Allele for tall (T) is dominant over allele for short height (t). So a cross between TT and tt will produce only tall offspring.  

Thus, an allele being dominant will determine frequency of occurrence of any specific phenotype.  

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