4. Important Terms Alleles – Genetic factors that controls a trait, Dominant = ability to mask the expression of a recessive allele (Capital) Recessive = masked by a dominant allele Homozygous - 2 identical alleles (TT, tt) Heterozygous - 2 different alleles (Tt) Genotype - genes of an organism = alleles received during fertilization. Phenotype = the physical appearance of a trait in an organism . Eg blue eyes, yellow flower