Biologia molecular

Environmental Influences on Gene Expression
By: Ingrid Lobo, Ph.D. (Write Science Right) © 2008 Nature Education 
Citation: Lobo, I. (2008) Environmental influences on gene expression. Nature Education 1(1)
Internal and external environmental factors, like gender and temperature, influence gene expression.
1. 1Introduction
2. 2Sex-Influenced and Sex-Limited Traits
3. 3Drugs andChemicals
4. 4Temperature and Light
5. 5References and Recommended Reading
 
The expression of genes in an organism can be influenced by the environment, including the external world in which the organism is located or develops, as well as the organism's internal world, which includes such factors as its hormones and metabolism. One major internal environmental influence that affects geneexpression is gender, as is the case with sex-influenced and sex-limited traits. Similarly, drugs, chemicals, temperature, and light are among the external environmental factors that can determine which genes are turned on and off, thereby influencing the way an organism develops and functions.
Sex-Influenced and Sex-Limited Traits
Sex-influenced traits are those that are expressed differently inthe two sexes. Such traits are autosomal, which means that the genes responsible for their expression are not carried on the sex chromosomes. An example of a sex-influenced trait is male-pattern baldness. The baldness allele, which causes hair loss, is influenced by the hormones testosterone and dihydrotestosterone, but only when levels of the two hormones are high. In general, males have muchhigher levels of these hormones than females, so the baldness allele has a stronger effect in males than in females. However, high levels of stress can lead to expression of the gene in women. In stressful situations, women's adrenal glands can produce testosterone and convert it into dihydrotestosterone, which can result in hair loss.
Sex-limited traits are also autosomal. Unlike sex-influencedtraits, whose expression differs according to sex, sex-limited traits are expressed in individuals of only one sex. An example of a sex-limited trait is lactation, or milk production. Although the genes for producing milk are carried by both males and females, only lactating females express these genes.
Drugs and Chemicals
The presence of drugs or chemicals in an organism's environment can alsoinfluence gene expression in the organism. Cyclops fish are a dramatic example of the way in which an environmental chemical can affect development. In 1907, researcher C. R. Stockard created cyclopean fish embryos by placing fertilized Fundulus heteroclitus eggs in 100 mL of seawater mixed with approximately 6 g of magnesium chloride. Normally, F. heteroclitus embryos feature two eyes; however, inthis experiment, half of the eggs placed in the magnesium chloride mixture gave rise to one-eyed embryos (Stockard, 1907).
A second example of how chemical environments affect gene expression is the case of supplemental oxygen administration causing blindness in premature infants (Silverman, 2004). In the 1940s, supplemental oxygen administration became a popular practice when doctors noticedthat increasing oxygen levels converted the breathing pattern of premature infants to a "normal" rhythm. Unfortunately, there is a causal relationship between oxygen administration and retinopathy of prematurity (ROP), although this relationship was unknown at the time; thus, by 1953, ROP had blinded approximately 10,000 infants worldwide. Finally, in 1954, a randomized clinical trial identifiedsupplemental oxygen as the factor causing blindness. Complicating the issue is the fact that too little oxygen results in a higher rate of brain damage and mortality in premature infants. Unfortunately, even today, the optimal amount of oxygenation necessary to treat premature infants while completely avoiding these complications is still not clear.
Yet another example of the way in which chemicals...