An organism’s phenotype can be influenced by several factors other than genotype, including genetic background and the environment. Sometimes, these effects can happen simultaneously.

The "genetic background" represents all of the related genes in a genome that may interact with the gene of interest.

The environment, on the other hand, encompasses many external factors that an organism may encounter, including temperature, diet, and light conditions.

Mutations in multiple related genes can affect the phenotype of an organism. For example, the brick red-eye color of wild type Drosophila is due to the presence of two pigments, ommochrome and drosopterin, which are regulated by three genes.

The vermilion gene is the first step in a pathway that produces the brown ommochrome pigment.

The brown gene encodes a membrane transport protein that is required for the production of the red pigment, drosopterin.

The white gene is responsible for carrying these pigments to the eye.

Mutations in the vermillion gene produce a fly with bright red eyes, while a mutation in the brown gene produces brown-eyed fly.

However, mutation in the white gene results in a white-eyed fly, even if the other two genes are functional.

 Temperature, one of many environmental factors, plays a vital role in determining the sex of developing European pond turtles.

When the eggs are incubated at 25˚C, gonadal tissues develop into testes and all of the offspring are male. When the eggs are incubated at 30˚C, however, gonadal tissues develop into ovaries and all of the offspring are female.

This period, in which gonadal tissue is responsive to temperature, is called the thermosensitive period.

An alteration occurs in gonadal tissue because the expression of Sox9 gene changes in response to temperature.

At lower temperatures, the expression is high, and at higher temperatures, the expression is repressed. This variable expression leads to observable changes in phenotype - offspring sex.