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).
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