Polygene

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A polygene is a gene that helps determine a trait together with many other genes. Each polygene adds a small effect, and the combined action of many such genes leads to polygenic inheritance—traits that are influenced by lots of genes and the environment, not by a single gene as in Mendel’s ideas.

Because many genes are involved, it’s hard to tell the effect of one gene alone. When scientists identify a gene that influences a trait, it’s called a quantitative trait locus (QTL). Polygenes often affect more than one trait (a phenomenon called pleiotropy).

Many common conditions, such as type 2 diabetes, are thought to involve many polygenes. In 2016, researchers noted a set of about 355 genes traced back to the last universal common ancestor of living organisms.

Polygenic traits show continuous variation rather than neat “yes/no” categories. Examples include height, skin color, eye color, and weight. A polygenic locus is any spot in the genome that contributes to the variation of such a trait.

Usually there are many genes, each with a small effect, so the overall distribution of these traits tends to form a bell-shaped curve. This idea follows from the Central Limit Theorem. But real life isn’t perfectly additive; interactions between genes (epistasis) can complicate the pattern.

To map polygenes, scientists use QTL mapping. By comparing known DNA markers with trait data and looking at linkage disequilibrium, they identify regions that explain a lot of variation. Sometimes a large region contains many genes responsible for the trait. A related concept is the expression QTL (eQTL), which changes how much mRNA and, therefore, protein an organism makes.

Researchers also study the genetic architecture of traits—whether many independent loci or a few loci, and how they interact—to understand evolution and variation in traits.