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Meiosis, the form of cell division unique to egg and sperm production, sets the stage for sex determination by creating sperm that carry either an X or a Y sex chromosome. But what is it about the X or Y that determines sex? Before a meiotic cell divides, its two sets of chromosomes come together and cross over, or swap, segments. The first animation shows normal crossing over, where the X and Y chromosomes exchange pieces only at their tips. The second animation shows a rare mistake in which the Y chromosome transfers a gene called SRY to the X chromosome, resulting in sex-reversed babies. Studies of sex-reversed individuals led researchers to identify the master switch for sex determination, the SRY gene, which tells a fetus to become a boy.

Meiosis, the form of cell division unique to egg and sperm production, sets the stage for sex determination by creating sperm that carry either an X or a Y sex chromosome. But what is it about the X or Y that determines sex? Before a meiotic cell divides, its two sets of chromosomes come together and cross over, or swap, segments. The first animation shows normal crossing over, where the X and Y chromosomes exchange pieces only at their tips. The second animation shows a rare mistake in which the Y chromosome transfers a gene called SRY to the X chromosome, resulting in sex-reversed babies. Studies of sex-reversed individuals led researchers to identify the master switch for sex determination, the SRY gene, which tells a fetus to become a boy.

The Y chromosome is only one-third the size of the X. Although the Y has a partner in X, only the tips of these chromosomes are able to recombine. Thus, most of the Y chromosome is inherited from father to son in a pattern resembling asexual, not sexual, reproduction. No recombination means no reassortment, so deleterious mutations have no opportunity to be independently selected against. The Y chromosome therefore tends to accumulate changes and deletions faster than the X. Degradation doesn't occur in X chromosomes because during female meiosis, the X has the other X as a full partner in recombination.

In this "clicker case," students learn about sex determination, meiosis, and chromosomal "crossing over" through the story of Santhi Soundararajan, an athlete from Kathakkurichi, India, who was stripped of a medal at the 2006 Asian Games after failing to pass a sex test. The case is called a clicker case because it combines the use of student personal response systems (clickers) with case teaching methods and formats. The case itself is a PowerPoint presentation (~2 MB) shown in class that is punctuated by questions students respond to using their clickers. It can be adapted for use without these technologies. Developed for an introductory biology class for both majors and non-majors, the case could also be used in an anatomy and physiology course or an endocrinology course.

In this "clicker case," students learn about sex determination, meiosis, and chromosomal "crossing over" through the story of Santhi Soundararajan, an athlete from Kathakkurichi, India, who was stripped of a medal at the 2006 Asian Games after failing to pass a sex test. The case is called a clicker case because it combines the use of student personal response systems (clickers) with case teaching methods and formats. The case itself is a PowerPoint presentation (~2 MB) shown in class that is punctuated by questions students respond to using their clickers. It can be adapted for use without these technologies. Developed for an introductory biology class for both majors and non-majors, the case could also be used in an anatomy and physiology course or an endocrinology course.

2:00 video It's not the Y chromosome itself that determines whether a baby is male or female, but rather a particular region called the SRY gene.

From something as small and complex as a chromosome to something as seemingly simple as the weather, sex determination systems vary significantly across the animal kingdom. Biologist and teacher Aaron Reedy shows us the amazing differences between species when it comes to determination of gender.

Explore inheritance when carried on the X chromosome with the Amoeba Sisters!

Animated primer of 75 key historical experiments in modern genetics

5:46 video, short quiz, open response