The term epigenetics refers to changes in phenotype (appearance) or gene expression caused by mechanisms other than changes in the underlying DNA sequence, hence the name epi- (Greek: over; above) -genetics.
There is no change in the underlying DNA sequence of the organism; the phenotype is expressed by activating some genes while inhibiting others. Epigenetics includes changes in gene function that occur without a change in the sequence of DNA. These changes occur as a result of the interaction of the environment with the genome.
Pregnancy is arguably the most critical period of developmental programming. New studies reinforce the importance of in-utero exposures (including dietary nutrients, microbial products, cigarette smoking, and certain maternal mediations) in fetal immune development and in programming the susceptibility to asthma and allergic disease.
Epigenetic determinants activate or silence fetal genes through alterations in DNA, histone methylation and acetylation.
DNA associates with histone proteins to form chromatin. Image source: Wikipedia, GNU Free Documentation License.
Histones are the chief protein components of chromatin. They act as spools around which DNA winds, and they play a role in gene regulation. Without histones, the unwound DNA in chromosomes would be very long. For example, each human cell has about 1.8 meters of DNA, but wound on the histones it has about 90 millimeters of chromatin.
Histone acetylation opens the chromatin to allow transcription of DNA. Histone deacetylation decreases gene expression. Deacetylation is reduced in chronic obstructive pulmonary disease (COPD).
From the NYTimes:
Genes as Mirrors of Life Experiences - Genes are far more than protein machines, pumping out their product like a popcorn maker.
Asthma and pregnancy: emerging evidence of epigenetic interactions in utero. Prescott SL, Clifton V. Curr Opin Allergy Clin Immunol. 2009 Oct;9(5):417-26.
Redon C, Pilch D, Rogakou E, Sedelnikova O, Newrock K, Bonner W (April 2002). "Histone H2A variants H2AX and H2AZ". Curr. Opin. Genet. Dev. 12 (2): 162–9.
In Scotland, passage of smoke-free legislation was associated with 18% decrease in asthma hospitalizations per year http://goo.gl/6LdK
Image source: Wikipedia, GNU Free Documentation License.