Two genetic possibilities for a fertilized egg.
If the two sex cells (gametes) that fuse during fertilization carry the same form of a gene for a specific trait, the organism is said to be a homozygote for that trait. If the gametes carry differing forms of the gene, the result is a heterozygote. Because genes may be either dominant or recessive (see dominance and recessiveness ), the genetic composition ( genotype ) of an organism cannot always be determined by the organism's physical appearance ( phenotype ).