Phenotype

A phenotype is the set of observable characteristics of an organism, such as its form, physiology and behavior. It results from the interaction of the organism's genes, its environment and the chance events of development. The term was introduced by Wilhelm Johannsen in 1909, along with genotype.

The genotype is the organism's set of alleles, and it sets a range of possible phenotypes, while the environment selects among them. Height depends on genes and on nutrition, and the color of a hydrangea depends on soil acidity. A graph of a genotype's phenotype across environments is called its reaction norm, and genetic effects and environmental effects may interact.

In Mendelian genetics, a dominant allele determines the phenotype in a heterozygote, as in round and wrinkled peas. Many traits, such as skin color or yield, are influenced by many genes and are continuous. Epigenetic marks can also change gene expression without changing DNA sequence, giving different phenotypes from the same genotype.

Natural selection acts on phenotypes, because it is the traits that determine survival and reproduction, and evolution happens when the heritable part of that variation changes across generations. Richard Dawkins extended the idea to the "extended phenotype," including effects of genes outside the body, such as a beaver's dam. See evolution.

• Gene — the unit of inheritance
• Natural Selection — the process that acts on phenotypes
• Epigenetics — regulation that changes phenotype without changing DNA