AP Syllabus focus: ‘
An organism’s genotype is its inherited allele set, whereas phenotype is the observable expression of those traits.’
Understanding heredity requires separating what an organism carries in its DNA from what you can observe in its structure and function. This distinction underpins predictions about traits, inheritance patterns, and biological variation.
Core idea: what you have vs what you show
Practice Questions
FAQ
They use measurable outputs such as enzyme activity, metabolite concentration, electrical activity, or gene expression readouts (e.g., protein abundance), treating these as phenotypic traits.
Penetrance is whether a genotype shows any phenotype at all in a population.
Expressivity is the degree or severity of the phenotype among individuals who do show it.
Cell types differ in which genes are switched on or off.
Different transcription factors and chromatin states lead to different proteins being produced, creating specialised cellular phenotypes.
Phenotype does not change inherited DNA sequence in most cases.
However, phenotypes can influence which genotypes are passed on indirectly via survival and reproductive success.
Different genotypes can look similar, and small genetic differences may have subtle phenotypic effects.
Also, measurement conditions and trait definitions can shift what is recorded as the “same” phenotype.
