If an organism inherits 1 dominant allele from one parent and 1 recessive allele from the other parent, what is it called?

heterozygous

Explanation:

An allele is a variant form of a gene. Some genes have a variety of different forms, which are located at the same position, or genetic locus, on a chromosome. Humans are called diploid organisms because they have two alleles at each genetic locus, with one allele inherited from each parent. Each pair of alleles represents the genotype of a specific gene. Genotypes are described  as heterozygous if the two alleles differ.