For each observable trait (phenotype), an organism inherits 2 alleles, one from each parent. These alleles make up the organism's genotype. Chromosomes that have the same genes (but possibly different alleles) are called homologous chromosomes.
If the 2 alleles at a locus (the region on a chromosome where a gene is found) are the same, the organism is homozygous, otherwise it is heterozygous, and the dominant allele determines the organism's phenotype.