In mammals, the bipotential gonad develops as an ovary unless SRY is expressed. If SRY is experimentally expressed in an XX mouse embryo's gonad, what develops?
AA normal ovary, because XX chromosomes override SRY
BA testis, because SRY is sufficient to initiate the testis-determining cascade regardless of chromosomal sex
CA mixed gonad with both testicular and ovarian tissue
DThe gonad degenerates because of the conflict between SRY and XX chromosomes
This experiment (transgenic XX mice carrying the Sry gene) was performed by Robin Lovell-Badge's group and demonstrated that SRY is sufficient for testis determination. The XX mice with SRY developed testes, male internal anatomy, and male external appearance (though they were infertile because spermatogenesis requires Y-linked genes beyond SRY). This proved that mammalian sex determination is a binary switch triggered by a single gene — SRY activates Sox9, which drives Sertoli cell differentiation and the downstream testis program, overriding the default ovarian pathway.
Question 2 True / False
Temperature-dependent sex determination (TSD) in reptiles demonstrates that genetic factors play no role in reptilian sex determination.
TTrue
FFalse
Answer: False
While incubation temperature is the primary sex-determining signal in many reptiles (higher temperatures produce females in some species, males in others), this temperature signal acts through the same molecular pathways (aromatase, Sox9, DMRT1, Foxl2) used in genetic sex determination. Temperature influences the expression or activity of these conserved sex-determination genes. Furthermore, some reptile species have genetic sex determination (GSD), some have TSD, and some have both. The molecular toolkit for gonad differentiation is conserved; what differs is the upstream trigger — a genetic signal (SRY, ZW) versus an environmental signal (temperature).
Question 3 Short Answer
Why must sex determination include active maintenance of the chosen sex throughout life, rather than being a single irreversible decision?
Think about your answer, then reveal below.
Model answer: Adult gonads continuously maintain their sexual identity through mutual antagonism between testis-maintaining and ovary-maintaining transcription factor programs. DMRT1 maintains testis identity by repressing Foxl2 (an ovarian transcription factor); Foxl2 maintains ovary identity by repressing Sox9 (a testis transcription factor). If DMRT1 is deleted in adult mouse testes, Sertoli cells transdifferentiate into granulosa-like cells and the testis begins to adopt ovarian features. If Foxl2 is deleted in adult ovaries, granulosa cells transdifferentiate into Sertoli-like cells. This reveals that sex determination is not a one-time event but an ongoing active process — adult gonadal identity requires continuous transcriptional reinforcement, and the alternative sexual program remains latent and ready to activate if the repressive barrier is removed.
This discovery (Matson et al., 2011; Uhlenhaut et al., 2009) was surprising because it overturned the assumption that adult cell fates are permanently fixed. It also has implications for disorders of sex development and for understanding how environmental endocrine disruptors might affect gonadal function.