An alleles is dominant is heterozygote when it is

An allele is dominant and heterzygote (Pp) when a dominant allele (P) is crossed with a recessive allele (p).

Explanation:

A dominant allele is the one with a particular dominant character or phenotype which dominates even though there are other alleles found.  

A heterozygous allele results from two different alleles coding for a gene.

A heterozygous dominant allele (Pp) results from the crossing of a dominant allele (P) with a recessive allele due to complete masking of the recessive allele (p).

For example, when dominant brown eyes are crossed with recessive blue eyes, in a heterozygous dominant allele results in brown eyes and masks the recessive blue eyes.