Editing Quantitative trait locus (section)

===Examples===
The above are well-known examples of diseases having both genetic and environmental components. Other examples involve atopic diseases such as [[atopic eczema|eczema]] or [[atopic dermatitis|dermatitis]],<ref name="Tissot"/> [[spina bifida]] (open spine), and [[anencephaly]] (open skull).<ref name="Proud">{{cite web  | author = Proud, Virginia  | author2 = Roberts, Helen  | name-list-style = amp  | title = Medical Genetics: Multifactorial Inheritance  | publisher = Children's Hospital of the King's Daughters  | date = 31 December 2005  | url = http://www.chkd.org/HealthLibrary/Content.aspx?pageid=P02134  | access-date = 6 January 2007  | archive-date = 15 October 2006  | archive-url = https://web.archive.org/web/20061015185017/http://www.chkd.org/HealthLibrary/Content.aspx?pageid=P02134  | url-status = dead  }}</ref>

While [[schizophrenia]] is widely believed to be multifactorially genetic by [[Biopsychiatry|biopsychiatrists]], no characteristic genetic markers have been determined with any certainty.{{cn|date=July 2024}}

If it is shown that the brothers and sisters of the patient have the disease, then there is a strong chance that the disease is genetic{{citation needed|date=March 2017}} and that the patient will also be a genetic carrier. This is not quite enough as it also needs to be proven that the pattern of inheritance is non-Mendelian. This would require studying dozens, even hundreds of different family pedigrees before a conclusion of multifactorial inheritance is drawn. This often takes several years.{{cn|date=July 2024}}

If multifactorial inheritance is indeed the case, then the chance of the patient contracting the disease is reduced only if cousins and more distant relatives have the disease.<ref name="Proud" /> While multifactorially-inherited diseases tend to run in families, inheritance will not follow the same pattern as a simple [[monohybrid cross|monohybrid]] or [[dihybrid cross]].<ref name="Clinical Genetics" />

If a genetic cause is suspected and little else is known about the illness, then it remains to be seen exactly how many genes are involved in the phenotypic expression of the disease. Once that is determined, the question must be answered: if two people have the required genes, why are there differences in expression between them? Generally, what makes the two individuals different are likely to be environmental factors. Due to the involved nature of genetic investigations needed to determine such inheritance patterns, this is not usually the first avenue of investigation one would choose to determine etiology.{{citation needed|date=August 2017}}

[[File:Example of QTL-Scan on a single Chromosom from PLoS Biology.jpg|thumb|right|A QTL for [[osteoporosis]] on the human chromosome 20]]