Editing Quantitative trait locus (section)

===Heritable disease and multifactorial inheritance===
A mutation resulting in a disease state is often recessive, so both alleles must be mutant in order for the disease to be expressed phenotypically. A disease or syndrome may also be the result of the expression of mutant alleles at more than one locus. When more than one gene is involved, with or without the presence of environmental triggers, we say that the disease is the result of multifactorial inheritance.{{cn|date=July 2024}}

The more genes involved in the cross, the more the distribution of the [[genotype]]s will resemble a [[normal distribution|normal, or Gaussian]] distribution.<ref name="Tissot" /> This shows that multifactorial inheritance is polygenic, and genetic frequencies can be predicted by way of a polyhybrid [[Mendelian inheritance|Mendelian]] cross. Phenotypic frequencies are a different matter, especially if they are complicated by environmental factors.{{cn|date=July 2024}}

The paradigm of polygenic inheritance as being used to define multifactorial disease has encountered much disagreement. Turnpenny (2004) discusses how simple polygenic inheritance cannot explain some diseases such as the onset of Type I diabetes mellitus, and that in cases such as these, not all genes are thought to make an equal contribution.<ref name="Turnpenny" />

The assumption of polygenic inheritance is that all involved loci make an equal contribution to the symptoms of the disease. This should result in a normal (Gaussian) distribution of genotypes. When it does not, the idea of polygenetic inheritance cannot be supported for that illness.{{cn|date=July 2024}}