Organisms can be homozygous or heterozygous for a gene. An organism is homozygous dominant, if it carries two copies of the same dominant allele. It is homozygous recessive, if it carries two copies of the same recessive allele.
Heterozygous means that an organism has two different alleles of a gene.
The genotype is the genetic makeup or constitution of an organism. The phenotype is the external or physical appearance of an organism.