I am reading about GWAS in heritability. They usually say to calculate heritability on a liability scale but after searching for this word "liability", I still don't understand clearly what does liability mean? Does anyone have any easy-to-understand explanation regarding this term? Thank you very much!
When you use linear mixed models to estimate heritability you assume that the underlying trait is normally distributed which is called a disease liability scale.
For continuous traits this is not a problem but for binary traits, this becomes an issue because you have a 0/1 value for a phenotype and usually there is a higher proportion of cases in the study sample than the general population prevalence of disease, which leads to an ascertainment bias.
The population prevalence also varies between populations, for example the prevalence of malaria in one continent is vastly different from another.
So to make the heritability estimates comparable and also to correct for the ascertainment bias, the observed scale heritabilities for dichotomous traits are transformed to the liability scale taking population prevalence and the case proportion into account.
Some good papers for reading:
Falconer D.S. The inheritance of liability to certain diseases, estimated from the incidence among relatives. Ann. Hum. Genet. 1965;29:51–71 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3059431/