Nikki and Thomas Araguz were married in 2008. Two years later, Thomas Araguz, a volunteer firefighter, was tragically killed in the line of duty. Last May a Texas judge nullified their marriage, denying Nikki her right to a widow's pension. His reason? Nikki Araguz was born a male, and under Texas law two men may not be legally wed.

Since 1996, Justin Graham Purdue has lived as a woman, undergoing surgery and hormone therapy shortly after she and Thomas were married. Yet Nikki carries within her cells one X and one Y chromosome. No surgery can alter that.

Nothing in biology is as fundamental as the distinction between male and female. However, those differences are actually quite small, even to the point where an intersex individual can be born with reproductive organs of both sexes.

Most believe that sex chromosomes determine our gender. If you are born with two Xs, you're a woman. Inherit an X and a Y, you're a guy.

Surprisingly, science shows us that this is not always the case. Individuals born with two X chromosomes and one Y may have a condition known as Klinefelter's syndrome. Anatomically male, they lack normal amounts of testosterone and develop differently. While true Klinefleter's is rare, 1-in-500 males carries an extra X chromosome yet live normal heterosexual lives.

British actress Caroline Cossey is one such XXY person. Born with male genitalia, in her teens Caroline realized that she was more female than male. At 18, the statuesque brunette adopted the name Caroline and landed a part as a bikini-clad Bond girl in "For Your Eyes Only." She was soon outed by a tabloid and her acting career was ruined. In 1983, a court denied Cossey permission to marry her Italian boyfriend. Largely through her public advocacy, the U.K. Gender Recognition Act became law in 2005 and now offers legal protections for people like Caroline.

Individuals with androgen insensitivity syndrome have had nature take them in a different direction. Androgen is a hormone that stimulates the development of a male fetus. For androgen to be a useful signal it must be perceived. The androgen receptor gene codes for a protein needed to properly process the androgen signal. A mutation in the AR gene can render the protein useless and the fetus may develop as a female. Out of the 3 billion DNA molecules that comprise the human genome, a single change can cause an XY fetus to develop as a female.

As one of the most prominent individuals to openly discuss her AIS, jazz singer Eden Atwood has wrestled with gender identity for years. With all of the female external parts but no uterus or ovaries, Atwood is every bit a woman, but with a man's genes.

We are still learning how subtle changes during critical stages of in-utero development can alter one's physical, and even psychological, perception of gender.

Recently, the U.S. Food and Drug Administration rejected the Natural Resources Defense Council's request to have bisphenol A banned from food containers.

Many know of the potential health risks associated with BPA, which led to its elimination from baby bottles and from all food packaging in Canada. What is less well-known is that in the 1930s, BPA was medically used as an estrogen replacement for women. The chemical structure of BPA and many chemicals found in pesticides, fabrics and plastics can act as hormone mimics. Studies have shown that these chemicals can alter the gender of developing offspring in many animals.

The Defense of Marriage Act defines marriage as being between one man and one woman. As politicians debate the legal status of DOMA, they should remember that the law relies on the unambiguous definition of terms. Science now shows us that the biological line between man and woman can be a very blurry one. As of last September, Australians may now select M, F, or X to indicate their gender on passports.

As one transgender woman who transitioned after being married to another woman put it, "We are two people who are each other's soul-mates. Gender shouldn't even be a consideration." In a country founded on the principles of personal freedom and the pursuit of happiness, shouldn't that be enough?

• Mark Farmer is professor of cellular biology at the University of Georgia.