A lot of interest about epigenetics and "epi-marks" or epigenetic markers has been caused by the recent finding that epi-markers seem to be responsible for the development of homosexuality.
Epigenetics is (the study of) the modification of gene expression (controlling whether a gene results in a functional product such as a protein) by factors that are external to the DNA sequence. Some definitions include heritability, so that any modification of gene expression is inherited by the offspring. But there are non-heritable forms of epigenetics, as shown in the classical example of bees (BBC news article link), where an environmental factor such as food source results in a change in gene expression.
For some basic details of epigenetics I will refer you to the well referenced Wiki page (Epigenetics), which also includes a bit about the etymology and definitions of the word.
The question remains whether the paper being discussed in all the news articles provides any evidence of the epi marks for homosexuality.
The paper itself can be found here - The Quarterly Review of Biology
It is a tough read. Essentially, the authors are making a hypothesis about a mechanism that may result in human homosexuality. They provide a review of the literature and make logical deductions from this in order to create a model. Biological modelling is an area I am not that familiar with, admittedly. However, in general, while the models can describe very nicely how something or a system might behave and make predictions that can be experimentally tested, the models themselves are not experimental evidence.
The hype about researchers showing that epi-marks cause homosexuality is a vast leap. It may be possible. It may be logical. Biology is rarely logical, however, and it needs to be tested. This is one of the stages of scientific experimentation.
1. Observations are made and evidence from previous experiments is gathered.
2. An hypothesis is proposed
3. Experiments that address the hypothesis are designed.
4. Experimental evidence is collected.
5. The validity of the hypothesis (or null hypothesis as often used in statistics) is assessed.
6. Hypothesis is accepted or rejected and conclusions drawn. The cycle starts again.
The paper is at step 2. It is an important hypothesis to make and will lead to interesting experiments. But it is not definitive by any means and certainly we cannot yet claim that epi marks are the cause of homosexuality.
One of the more disturbing factors about that is the apparent need for there to be a cause for sexuality. It does not address a wider acceptance of homosexuality, nor does it provide solutions for those trying to defeat bigotry and gain legal and moral equality. Do we need a biological cause to except homosexuality as a valid and moral form of sexuality?
As debate over recent proposals for legislation over "gay marriage" has reached a peak (BBC Q and A on current gay marriage issues), the timing of this paper probably couldn't have been better for gaining media attention. However, as usual beware of claims made in such articles, they may not accurately reflect the science in the background.
Skeptical kinkster musing on whatever takes my fancy!