With genetics and gender, the science is not so simple


Gender identity has become a controversial issue in our society. Biologically, gender and its expression are more complex than a simple binary classification scheme. Whether we are male or female is largely controlled by our sex chromosomes. Those who are physically female usually have two X chromosomes (XX), and those who are physically male usually have one X and one Y chromosome (XY). However, there are exceptions to this simple relationship.

Androgen insensitivity syndrome (AIS) is the clearest example of a lack of connection between sex chromosomes and physical appearance of sex. Chromosomal males (XY) with AIS have a gene on the X chromosome that results in a partial or total inability of cells to respond to androgens. Androgens are steroid hormones that regulate the development and maintenance of male characteristics. This unresponsiveness to androgens can impair or prevent the development of male genitalia, as well as impair or prevent the development of male secondary genitalia at puberty. Thus, XY individuals with AIS are usually classified at birth as female even though they are genetically male. The effects of AIS vary among individuals. Those with complete AIS have a normal female body shape but lack a uterus and have a shallow vaginal cavity.

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AIS is rare. Approximately 1 in 20,000 births to XY individuals is affected. However, in our country of over 300 million people, there are nearly 10,000 people with AIS.

There are many other genetic conditions where an individual’s chromosomal sex does not match their male or female appearance. For example, XX male syndrome is an intersex condition in which an individual with XX chromosomes has male characteristics. This condition is usually caused by the transfer of the region of the Y chromosome responsible for initiating male development to the X chromosome. There are approximately 6,000 people in the US with this condition.

Intersex individuals have different combinations of male and female sex characteristics, including chromosomes and sex organs. In addition to genetic causes, environmental effects can lead to intersex individuals. For example, exposure to certain hormones during embryonic development can create people whose physical sex does not match their genetic sex.

According to the American Medical Association, approximately 1.7 percent of births in the U.S. deviate in some way from the simple gender binary. Other estimates of the proportion of intersex births are much lower (as low as 0.02%), depending on the definition of intersex. Therefore, there are approximately 100,000 to 5 million intersex people in the US.

The recognition of intersex people creates policy challenges in a society that has long been based on a simple classification of male or female. Science does not provide answers to how to solve these complex problems, but rather science provides the factual information needed to make these societal decisions. Understanding the scientific basis of gender through this information is necessary to develop appropriate and just public policies.

Fred Allendorf is professor emeritus of biology at the University of Montana and writes about current research for the Missoulian.

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