Editing Genetics (section)

=== Multiple gene interactions ===
[[File:Galton-height-regress.png|thumb|right|Human height is a trait with complex genetic causes. [[Francis Galton]]'s data from 1889 shows the relationship between offspring height as a function of mean parent height.]]

Organisms have thousands of genes, and in sexually reproducing organisms these genes generally assort independently of each other. This means that the inheritance of an allele for yellow or green pea color is unrelated to the inheritance of alleles for white or purple flowers. This phenomenon, known as "[[Mendelian inheritance#Law of Independent Assortment (The "Second Law")|Mendel's second law]]" or the "law of independent assortment," means that the alleles of different genes get shuffled between parents to form offspring with many different combinations. Different genes often interact to influence the same trait. In the [[Omphalodes verna|Blue-eyed Mary]] (''Omphalodes verna''), for example, there exists a gene with alleles that determine the color of flowers: blue or magenta. Another gene, however, controls whether the flowers have color at all or are white. When a plant has two copies of this white allele, its flowers are white—regardless of whether the first gene has blue or magenta alleles. This interaction between genes is called [[epistasis]], with the second gene epistatic to the first.<ref name=griffiths2000sect644>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.644 |chapter=Gene interaction and modified dihybrid ratios}}</ref>

Many traits are not discrete features (e.g. purple or white flowers) but are instead continuous features (e.g. human height and [[Human skin color|skin color]]). These [[complex traits]] are products of many genes.<ref>{{cite journal | vauthors = Mayeux R | title = Mapping the new frontier: complex genetic disorders | journal = The Journal of Clinical Investigation | volume = 115 | issue = 6 | pages = 1404–1407 | date = June 2005 | pmid = 15931374 | pmc = 1137013 | doi = 10.1172/JCI25421 }}</ref> The influence of these genes is mediated, to varying degrees, by the environment an organism has experienced. The degree to which an organism's genes contribute to a complex trait is called [[heritability]].<ref name=griffiths2000sect4009>{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W. H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.4009 |chapter=Quantifying heritability}}</ref> Measurement of the heritability of a trait is relative—in a more variable environment, the environment has a bigger influence on the total variation of the trait. For example, human height is a trait with complex causes. It has a heritability of 89% in the United States. In Nigeria, however, where people experience a more variable access to good nutrition and [[health care]], height has a heritability of only 62%.<ref>{{cite journal | vauthors = Luke A, Guo X, Adeyemo AA, Wilks R, Forrester T, Lowe W, Comuzzie AG, Martin LJ, Zhu X, Rotimi CN, Cooper RS | title = Heritability of obesity-related traits among Nigerians, Jamaicans and US black people | journal = International Journal of Obesity and Related Metabolic Disorders | volume = 25 | issue = 7 | pages = 1034–1041 | date = July 2001 | pmid = 11443503 | doi = 10.1038/sj.ijo.0801650 | doi-access = free }}</ref>