Himalayan rabbits have white fur, but black feet, ears, and tail. The black pigment will only develop on parts of the body that are at a low temperature. Himalayan rabbits clearly possess the gene for making black fur, but only in the right environment will the (black fur) develop
— Bill Indge

Gene Expression: the process by which the information encoded in a gene is used to direct the assembly of a protein molecule. (NHGRI)

The ‘turning on’ of a gene. Most human genes are active, or ‘turn on,’ only in certain cells under certain conditions. Genes for eye color are active in eye cells but not in stomach cells. Similarly, some genes may lie dormant for years and then turn on late in life. (GNN) Some genes, such as those involved in the production of energy, are always expressed because the “proteins” they (code for) are essential for survival. Genes are also expressed or "repressed" in response to signals from within the body or from the “environment.” (Kandel, 257) "DNA" codes for small "RNA" molecules that, like switches, play a critical role in how and to what extent genes are used. (Venter, 40) Along with shaping our bodies and coloring our hair, genes constantly alter our brains by responding to experience. Genes create “messengers” that activate other genes that build proteins that activate or control yet other genes. This is part of the process that creates new “synapses.” (SAM, Oct/Nov07, 33-35) Environment can determine the expression of genes. (Norman, 7/21/09) Also referred to as 'activating,' ‘expressing,’ ’inducing,’ 'turning on,' and ‘switching on’ a gene.

Expressivity: describes degrees of expression of a single gene. The severity or extent (of gene expression). (Lewis, 93)

Penetrance: the degree or probability to which a “genotype” is expressed as a “phenotype.” (NCIt) The probability of a gene or genetic “trait” being expressed. The percentage also may change with the age range of the population. (HGPIA) The proportion of individuals with a “mutation” causing a particular disorder who exhibit clinical symptoms of that disorder. (GeneReviews) Describes degrees of expression of a single gene. The all or none expression of a genotype. (Lewis, 93) Editor’s note - if 80 of 100 people who inherit a dominant “allele” have its trait, the genotype is ’80% penetrant.’  This would be considered ‘highly penetrant.’

Completely Penetrant: gene or genes for a trait expressed in all the population who have the genes. (HGPIA) (Indicates) clinical symptoms are present in all individuals who have the disease-causing mutation. (GeneReviews) An allele combination that produces a phenotype in everyone who inherits it. (Lewis, 93) Editor’s note - an example is “Huntington’s disease.”

Incompletely Penetrant: genetic trait expressed in only part of the population. (HGPIA) Clinical symptoms are not always present in individuals who have the disease-causing mutation. (GeneReviews) A genotype that results in some individuals not expressing the phenotype. (Lewis, 93) Editor’s note - an example is “polydactyly.”

Pleiotrophy: multiple, often seemingly unrelated, physical effects caused by a single altered gene or pair of altered genes. (GeneReviews) One gene that causes many different physical traits such as multiple disease symptoms. (HGPIA) The expression of any given gene influences the expression of many other genes in the genome. Pleiotrophy is revealed when researchers study the effects of gene mutation. (Brooker, 343) When more than one phenotype is controlled by a single gene, that gene is said to be pleiotropic. In cats, for example, the genetic makeup that results in white fur color can also lead to deafness. (Mikulecky, 233) Pleiotrophy occurs when a single protein affects different body parts, participates in more than one biochemical reaction, or has different effects in different amounts. (Lewis, 95) Editor’s note - examples include “Cystic fibrosis,” “Marfan syndrome” and “porphyria variegata.”

Variable Expression: variation in clinical features (type and severity) of a genetic disorder between individuals with the same gene alteration, even within the same family (GeneReview) A phenotype (where) symptoms vary in intensity among different people. (Lewis, 93)