You can always tell a person's genotype from their phenotype.

Question

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While a person's phenotype may give some indication of their genotype, it is not always possible to determine a person's exact genotype just by observing their physical characteristics. This is because some phenotypic traits can be influenced by multiple genotypes or environmental factors. Additionally, people can have the same phenotype despite having different genotypes (known as phenotypic plasticity).

You Can Always Tell a Person's Genotype from Their Phenotype