This interactive tool demonstrates the influence of genotypic effect sizes and genotype frequencies on heritability at a single locus (with two alleles).

Overall heritability of a trait (which can be estimated using various family designs) is the aggregation of the effects of all loci influencing the trait, including interactions between loci.

The dark blue squares on the graph, the effect sizes of the genotypes, represent the average phenotypic value for all individuals with this genotype (they can be adjusted up and down).

The size of the circle surrounding the genotype effect reflects the frequency of this genotype, assuming Hardy-Weinberg proportions. These can be modified by changing the allele frequency (using the red slider).

The red points along the line represent the additive components of the genetic effects (subtract the overall mean from these and you have the breeding values). It is these values that are important for calculating additive variance and narrow sense heritability.

Genotypic, additive, and environmental variances are all shown on the same scale. Environmental variance can be modified (using the blue slider) to be up to four times the maximum possible amount of genotypic variance (in real life it can be much greater than this). Genotypic and additive variances are determined by the genotypic effect sizes and the genotype frequencies.

The scale of the genetic variances in this tool is arbitrary . in real life they will depend on the values of the phenotype (which in this tool range from 0 to 1).