Gregor Mendel famously used pea plants to show how single genes control characteristics, but in humans and most other organisms, inheritance is rarely that simple. The vast majority of an organism's observable features, known as its phenotype, are the result of multiple genes interacting rather than the inheritance of a single gene.
All genetic variants (also known as alleles) arise from mutations, but most phenotypic features are the result of the cumulative effect of many such variants. Because a student's genotype for a trait like eye colour involves several different genes, it is too complex to be modelled using a single Punnett square. Punnett squares are exclusively used to model single gene (monogenic) inheritance. Note that you are not expected to describe the exact mechanisms of how these multiple genes interact (such as one gene masking another, which is a process you do not need to know).
Students often state that "every feature is controlled by multiple genes" — be careful, as a few features (like blood groups) are controlled by a single gene, so always state that "most" features are polygenic rather than "all".
In multiple-choice questions, examiners expect you to recall that single-gene inheritance (like Mendel's peas) is the exception rather than the rule for most human traits.
Never try to draw a single Punnett square to model a polygenic trait like height or eye colour; always explain that there are too many possible allele combinations to model it this way.
Phenotype
The observable characteristics of an organism, resulting from the interaction of its genotype and its environment.
Genotype
The combination of alleles (genetic variants) an organism has for a particular gene or genes.
Monogenic inheritance
The determination of a characteristic by the inheritance of a single gene.
Polygenic inheritance
The determination of a characteristic by the interaction of multiple genes.
Continuous variation
Variation within a population in which a characteristic has a wide range of values that flow into each other with no distinct categories.
Discontinuous variation
Variation that produces distinct, discrete categories for a characteristic with no intermediate values.
Genetic variants
Different versions of a gene, also known as alleles, which arise from mutations.
Mutations
Changes in the DNA sequence that lead to the creation of new genetic variants.
Alleles
Different versions of the same gene (used interchangeably with genetic variants).
Put your knowledge into practice — try past paper questions for Biology B
Phenotype
The observable characteristics of an organism, resulting from the interaction of its genotype and its environment.
Genotype
The combination of alleles (genetic variants) an organism has for a particular gene or genes.
Monogenic inheritance
The determination of a characteristic by the inheritance of a single gene.
Polygenic inheritance
The determination of a characteristic by the interaction of multiple genes.
Continuous variation
Variation within a population in which a characteristic has a wide range of values that flow into each other with no distinct categories.
Discontinuous variation
Variation that produces distinct, discrete categories for a characteristic with no intermediate values.
Genetic variants
Different versions of a gene, also known as alleles, which arise from mutations.
Mutations
Changes in the DNA sequence that lead to the creation of new genetic variants.
Alleles
Different versions of the same gene (used interchangeably with genetic variants).