An organism is heterozygous for a gene that follows Mendels law of dominance which allele will be expressed as its phenotype

The dominant allele

Explanation:

The dominant allele will be expressed because in a heterozygous genotype, the phenotype will always show the dominant gene (in Mendelian genetics)

This is because the dominant allele will dominate the recessive one and it will "mask" the recessive allele.