Phenotype
A phenotype is the observable characteristics of an organism, produced by its genotype interacting with the environment.
Definition
A phenotype is what an organism actually looks like or how it behaves, the trait you can observe, such as brown eyes or a tall pea plant, and it results from the interaction between an organism's genotype and its environment. For a trait with complete dominance, the homozygous dominant and heterozygous genotypes produce the identical phenotype, so a monohybrid cross gives a $3:1$ phenotypic ratio even though the underlying genotypic ratio is $1:2:1$. More formally, phenotype spans every expressed characteristic from the molecular (protein abundance, metabolite levels) to the organismal (morphology, physiology, behavior), summarized in quantitative genetics as $V_P = V_G + V_E + V_{GE}$, and the capacity of one genotype to produce different phenotypes across environments, called plasticity, is described by a reaction norm; selection acts directly on phenotypes, and the response to selection depends on how much of that variance is additive genetic variance.
Example
Hydrangea plants with identical genotypes bloom blue in acidic soil and pink in alkaline soil, and Siamese cats carry a temperature-sensitive allele that darkens fur only on cooler body parts such as the ears and tail. The breeder's equation $R = h^2 S$ predicts that selecting parents $10\ \text{cm}$ above the population mean in height, with heritability $h^2 = 0.5$, yields offspring averaging $5\ \text{cm}$ taller, and phenylketonuria has a severe phenotype on a normal diet but a nearly normal one when phenylalanine is restricted from birth.
Key Insight
Identical twins share a genotype but not an exact phenotype, since fingerprints and even some disease outcomes differ because of environment and chance. Genes set a range of possible outcomes rather than a fixed destiny, which is why medicine, nutrition, and education can shift outcomes without ever changing DNA.