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Environmental Effects on Gene Expression

The development and expression of traits also depends on the external and internal environment. The frequency with which a certain trait is expressed in a population is called penetrance, and the extent to which a phenotype is expressed in an individual is called expressivity.
The external environment can influence phenotypes. For example, Himalayan rabbits have mostly white fur, while black coat color is found only on the extremities (ears, paws, nose and tail). The gene that produces pigment in the fur is only active at low temperatures, producing the unusual pattern.
Internal factors can influence the expression of genes. Age will often influence the expression of a gene, such as is seen with the development of Huntington’s disease in humans. Sex can also determine phenotypes. This is most evident in the preponderance of baldness in human males.
Other factors, such as behavior and intelligence, are also heavily influenced by environment. Although there is currently a debate among scientists (one that may not end for a long time), it is evident that these traits are determined by both “nature” (genetics) and “nurture” (environment).

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