phenotype

Meaning: Phenotype is the observable or measurable expression of a trait, such as seed shape, blood type or behavior. It arises when the genotype (the alleles carried) is expressed as proteins, and the environment can modify that expression; nutrition affects height, and temperature affects coat color in Himalayan rabbits. Allele interactions matter too: complete dominance, incomplete dominance (an intermediate heterozygote) and codominance (both alleles expressed, as in AB blood).

On the MCAT: Punnett squares require converting genotype ratios to phenotype ratios. For Aa × Aa with complete dominance, genotypes are 1:2:1 but phenotypes are 3:1; a dihybrid cross of two heterozygotes gives 9:3:3:1 phenotypes. In Hardy-Weinberg problems, the frequency of a completely recessive phenotype equals q², which lets you solve for q. Natural selection acts on phenotypes, and passages may compare organisms with identical genotypes in different environments.

Common confusion: A dominant phenotype can come from AA or Aa, so a test cross with a homozygous recessive is used to tell them apart. Dominant also does not mean common or advantageous; it describes which phenotype appears in a heterozygote, not how frequent the allele is.