Pleiotropy

=Pleiotropy=

**Pleiotropy** is the phenomenon where one gene has multiple different phenotypes associated with it. For instance, the gene may code for a product that is used in many cells, or for a signalling molecule that has a widespread endocrine effect.

An example of a pleiotropic gene is one in mice responsible for coat colour. The dominant allele for this gene is **Y**, for yellow coat colour, and the recessive is **y**, for agouti coat colour. The genotype **yy** gives agouti mice and the genotype **Yy** gives yellow mice. However, the genotype **YY** never surfaces phenotypically because the Y allele, when available in two copies, also causes lethality (premature termination of the embryo). Thus, Y has a pleiotropic effect: coat colour and lethality.

Pleiotropy can be problematic when selection favours one allele of the gene for one trait, and the other allele of that gene for another trait. **Antagonistic pleiotropy** refers to a gene that has a mixture of beneficial and detrimental phenotypic effects. This idea is related to the theory of ageing: that increased fitness in youth (for reproduction) causes decreased fitness in old age. For instance, a gene codes for the protein p53 which suppresses cell proliferation and thereby prevents cancer. However, it also prevents stem cell proliferation, hence the inability to replenish worn-out tissues later in life.

Pleiotropy is distinct from **polygenic traits**, where one phenotype is associated with multiple different genes.