In this Medium article, biochemist and writer Julia Serano discusses the myth of a binary biological sex, and its implications for oppression and social justice.
Scientific American: "Beyond XX and XY"
A host of factors figure into whether someone’s biological sex appears female, male, or somewhere in between. This reference poster from Scientific American represents some elements in the spectra of gender (identity) and (biological) sex.
Though this poster uses the phrase "intersex conditions", we advise using "intersex traits" instead.
Creator Amanda Montañez tells the story behind the infographic in her article, "Visualizing Sex as a Spectrum".
Gynandromorph Animals - BBC Earth article
This BBC Earth article explores the rare phenomenon of bilateral gynandromorphs, or animals which develop different physical sex characteristics on their left and right side. This occurs when a cycle of meiosis yields multiple viable eggs rather than one egg and three polar bodies to be discarded. When two eggs are fertilized by sperm, mosaicism can occur in the sex chromosomes.
When introducing students to the article, the teacher should be sure to note that overgeneralizing language is briefly used to describe humans (“In humans, men have an X and a Y chromosome, while women have two X chromosomes.”) However, the article can still have value in demonstrating the nonbinary nature of biological sex and the scientific processes by which evidence is gathered to support this.
A Reddit post shows a gynandromorph lobster found in the wild:
Sex Estimation Based On Multiple Pelvic Indicators More Accurate, Based On 49 19th c. Canadian adults — Bass Lab & Field Manual
The most accurate single indicator was sacrum shape (94.1%). On the other hand, three combinations of pelvic criteria produced higher levels of accuracy than the trait list as a whole: obturator foramen shape and presence of the ventral arc (98%); obturator foramen shape and presence of the ventral arc (98%); obturator foramen shape and true pelvis shape (98%); pubic shape and acetabulum shape and size (95%).
The William Bass Field Manual mentions other studies of European populations, but reproduces in full several tables by Rogers and Saunders based on 49 “adult skeletons from a 19th-century cemetery located on the grounds of the St Thomas Anglican Church in Belleville, Canada.”
Embedded below are tables for:
Table 3-24. Accuracy Levels for Each Individual Pelvic Trait (after Rogers & Saunders (1994:1051, Table 5) (Bass 2005: 216)
Table 3-25. Probability of Estimating Sex Correctly for Combinations of Two Traits (after Rogers & Saunders (1994:1051, Table 3) (Bass 2005: 216)
Curious about what the broad labels for male and female anatomical regions are for those anthropologists using the Bass Manual?
Source
Bass, William. 2005. Human Osteology: A Laboratory and Field Manual, 5th ed. Special Publication No. 2 of the Missouri Archaeological Society. Columbia, MO. http://coas.missouri.edu/mas
Sexing skeletons: the biology of sex, gender, and the incredible diversity of human biology
In this blog post, Gender-Inclusive Biology contributor Lewis Maday-Travis shares a presentation he created to wrap-up a unit on the skeletal system. The presentation focuses on the science of the intersex experience and nonbinary identity, both of which are supported by scientific research yet frequently erased in public discourse.
Diagram of Mammalian Sex Determination
Image shows the development of testes or ovaries from a bipotential gonad in the first 20 days after conception. Factors shown to be influencing gonadal development include the Sry and Sox9 genes, FGF9, Antimullerian hormone, PGD2, WNT4, RSPO1, FOXL2, and Follistatin.
This diagram shows basic gene expression and hormone signalling pathways leading to to development of the mammalian gonads. The “dpc” on the x-axis of the graph means days post-conception. This image goes one step further than teaching that “the Y chromosome causes development of testes.”
For an AP Biology class learning about signaling pathways, reading a larger selection from the source article may be useful.
Sex Determination – Why So Many Ways of Doing It?
Image shows simplified diagrams of some systems of sex determination - hermaphroditism in plants and fish, environmental sex determination in reptiles and plants, and genotypic sex determination in humans and birds.
This PLOS Biology article details the diverse mechanisms by which sex is determined in different species, how they evolved, and how they may look in the future. The article opens with a treatment of three common myths of sex determination.
Editor’s note: The term "hermaphrodite" is appropriate for referring to non-human animals with sex characteristics that do not fit typical binary notions of male or female bodies. For humans, “intersex” is the appropriate term—learn more here!
Although the entire reading is not for k-12 audiences, the three myths, Figure 3, Figure 4, and Box 2 are suitable for high school.
Article: "Stop Using Phony Science to Justify Transphobia"
Image shows a hand holding up a speech bubble with the transgender pride flag colors - blue, pink, and white. The speech bubble contains the transgender symbol with an equal sign inside.
In this Scientific American article, Simón(e) D Sun explains how misunderstandings of science have been used to dehumanize trans people in society. Sun explains how genetic sex and the relationship between the body, brain, and hormones are both complex and validate the existence of transgender people.
Video: There Are More Than Two Human Sexes
This SciShow video summarizes the complexity of human sex determination, which often wrongly thought to be binary. Mosaicism, chimerism, and intersex traits and nonconsensual surgeries are mentioned.
Activity: Sex Verification of Athletes
Drawing of Indian professional sprinter Dutee Chand running a race.
In this web activity by HHMI BioInteractive, students integrate knowledge from several areas of genetics to understand the history and the science behind sex verification of athletes.
The key concepts covered in the activity are:
Information stored in DNA is expressed as RNA and proteins, which impact anatomical structures and physiology.
Many gene products determine the development of biological sex characteristics in humans.
Like many traits, biological sex consists of a spectrum of phenotypes.
Scientific knowledge (e.g., knowledge about which genes and other factors contribute to a person's traits) can inform societal practices (e.g., what types of tests can be used to define an individual's biological sex), although ultimately implementing those practices reflects our values as a society.
The lab techniques covered in the activity are:
Physical examination
Barr body staining
PCR and gel electrophoresis of SRY gene
Serum testosterone measurement