The allele that is expressed when two different alleles occur together is called the dominant allele.

Here's a breakdown:

* Alleles: Different versions of a gene.

* Dominant allele: An allele whose trait is expressed even if only one copy is present. It masks the effect of the recessive allele.

* Recessive allele: An allele whose trait is only expressed if two copies are present. It is masked by the dominant allele.

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