Sex and gender are binaries? Sorry, that's a scientific falsehood

In this SF Chronicle piece, Ash Zemenick discusses evidence for biological sex as a continuum rather than a binary. They argue that humans whose chromosomes, gametes, or hormones do not fit into a binary are common and that it is more useful to view them as a form of diversity rather than as an exception to a rule.

Differences in Sex Determination Investigation

Alex Maes created this investigation which includes a variety of videos and infographics also featured on this website. Two versions are available, one more scaffolded than the other.

Alex writes:

I've made a lesson like this too using lots of resources from genderinclusivebiology.com, as well, as some of my own. I have an individual, less scaffolded version I do with my IB juniors, and a more scaffolded version with whole class, individual, and small group components I did with my freshman! Feel free to use!

Edit (6/8/23): Thank you to Wayne for noting the lesson materials include diagrams including humans in a chart entitled hermaphrodites. It is important to discuss with students how language changes when describing human versus non-human species because of how “hermaphrodite” was historically used as a pejorative to stigmatize people. Wayne provided the link embedded in the disclaimer most frequently seen on related GIB posts: 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! (-RXS)

The Intersex Roadshow (Blog)

In this blog, intersex sociologist Dr. Cary Gabriel Costello writes about current intersex issues in historical context.

Intersex people are supposed to lie low and keep quiet. Not me.

I'm not defective, I'm not disordered, I'm not ashamed. I just don't fit in your M/F boxes.

I'm intersex by birth and honest by choice.

Sex Determination and Non-disjunction Lesson by Sabrina Kayed

This highly accessible lesson was designed by Sabrina Kayed for Grade 11 Biology. The lesson is designed to move students’ understanding of biological sex beyond the simplistic binary model and introduce them to the idea of sex as a spectrum.

Inclusive Zines from the Queer Sex Ed Community Curriculum

The Queer Sex Ed Community Curriculum is an LGBTQ-led project that is developing inclusive, trauma-informed, and sex-positive resources for use with youth. Their resource library includes zines, posters, and training materials that you can implement in your classroom. Check out the awesome Sex Diversity in Nature Zine, as well as their zine on Deconstructing the Gender Binary.

Sex Redefined: The Idea of 2 Sexes Is Overly Simplistic

In this Scientific American article, Claire Ainsworth, discusses evidence for the spectrum model of biological sex using introductory language.

Article: Transgender People and “Biological Sex” Myths

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".

Teaching about Gender Diversity (Book)

Written by teacher for teachers, Teaching about Gender Diversity is an edited collection of interdisciplinary lesson plans that provides K–12 teachers with the tools to implement gender-inclusive practices into their curriculum and talk to their students about gender and sex. Edited by Susan W. Woolley and Lee Airton, this engaging collection features teachers' perspectives on teaching about gender from across North America. Divided into three sections dedicated to the elementary, middle, and secondary grade levels, this practical resource provides lessons for a variety of subject areas, including English language arts, STEM, and health and physical education. To view more titles in Education, visit canadianscholars.ca.

Temperature of egg determines gonads that form in turtles. (Gender Showcase, 9-12)

Discussion questions:

1. How do changing temperatures affect a sea turtle egg’s development?

2. How will average global temperature changes affect a sea turtle egg’s development?

References

• M. Ewert, D. Jackson, and C. Nelson, 1994, Patterns of temperature-dependent sex determination in turtles, J. Exp. Zool. 270:3-15.

• Roughgarden p. 203.

The temperature of the nest determines the gonads that form in crocodile eggs. (Gender Showcase, 9-12)

DISCUSSION QUESTIONS:

1. How do changing temperatures affect a crocodile egg’s development?

2. How will average global temperature changes affect a crocodile egg’s development?

3. Predict an increase, decrease, or stable population change if average temperatures increase or decrease.

Book excerpt: Among reptiles, specifically turtles, crocodiles, and some lizards, gonadal identity is determined by the temperature at which eggs develop, not by chromosomes. The eggs are usually laid in the ground and covered with sand or moist dirt from which they absorb water, swelling in size as they age. Reptile embryos start developing within their egg, and after a while primordial germ cells form. When reptile primordial germ cells move to the genital ridges of their parents, both the germ cells and the parental embryo presumably experience the same environmental temperature. Both germ cells and parent therefore receive the same message about which sex to develop as, and their agendas automatically agree.

Scientists reported the first case of intersexuality in an African dwarf crocodile (Osteolaemus tetrapspis), a 10 year-old male-presenting crocodile with gonads that were ovotestes.

We used to think crocodiles couldn’t be intersex, because we had never found any before. Our understand was that crocodiles needed two separate types of cells that never occur together.

This discovery shows we have so many more questions to ask about sex determination!

Image caption: A dwarf crocodile. (C) Jim Frazee

Because the model we use to explain sex determination in crocodiles cannot help explain this evidence, we must keep asking questions and build better models for looking at our evidence.

Langer: Half of the 22 extant species of crocodilians have been examined for occurrence of temperature dependent sex determination (TSD). In TSD reptiles, masculinizing temperatures yield 100% or a majority of males, whereas feminizing temperatures yield 100% or a majority of females. In the transition range of temperature (TRT), a mix of males, females and sometimes intersexes are obtained. However, the molecular mechanisms behind TSD and an explanation for the occurrence of intersexuality remain elusive.

References

• C. Johnston, M. Barnett, and P. Sharpe, 1995, The molecular biology of temperature-dependent sex determination, Phil. Trans. R. Soc. Lond., ser. B, 350: 297-304.

• J.W. Lang and H. Andrews, 1994, Temperature-dependent sex determination in crocodilians, J. Exp. Zool. 270-28-44.

• S. Langer, K. Ternes, D. Widmer, & Frank Mutschmann. The first case of intersexuality in an African dwarf crocodile (Osteolaemus tetraspis). Zoo Biol. 33:459–462, 2014. DOI:10.1002/zoo.21149

• C. Smith and J. Joss, 1993, Gonadal sex differentiation in Alligator mississippiensis, a species with temperature-dependent sex determination, Cell Tissue Res. 273:149-62. 

• Wibbles, Bull, and Crews, 1994, Temperature-dependent sex determination. Journal of Experimental Zoology 270(1):71 - 78. DOI: 10.1002/jez.1402700108

Diagram of Mammalian Sex Determination

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.

Source: Kashimada, Kenichi, and Peter Koopman. "Sry: the master switch in mammalian sex determination." Development 137.23 (2010): 3921-3930.

Sex Determination – Why So Many Ways of Doing It?

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"

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: Touch-Based Sex Determination in Slipper Limpets

Slipper limpets are unique because their sexual development is influenced by touch with other individuals of the same species. Maryna Lesoway from the University of Illinois at Urbana Champaign concisely explains current research on the phenomenon.

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.

Video: The Crocodile's Unusual Sex-Determination System Explained

This video from Facts in Motion illustrates how the sex of crocodiles is determined by environment and not genetics. The sex ratio of developing crocodile eggs is shown to be a function of temperature, thought to be mediated by a thermosensor protein. Climate change may impact or even cause the extinction of crocodile populations.

This sex-determination system is not so “unusual” because it occurs in other reptiles and fish species. But it is certainly interesting and gathering attention. The College Board’s 2018 AP Biology exam had a free response question about temperature-dependent sex determination in fish.

[HS and College] Inclusive and Accurate Approaches for Teaching Sex and Gender in Biology

Project Biodiversify team members Alex Webster, Ash Zemenick, and Sarah Jones presented this workshop at the 2018 Annual Meeting for the Ecological Society of America (ESA). The slides contain comprehensive information about the benefits of teaching inclusive biology, and how to adapt existing curricula using an iterative process. Extensive examples are given for sexual reproduction, sex determination, and sexual selection.