The university's study employs mutated SRY genes shared by a father and a sterile XY daughter. Females usually develop with an XX pair, but, in these families, the father instead produced a daughter with an XY pair. This occurs during fetal development when the SRY gene's master switch fails to trigger. Internal female tissues, such as the uterus and fallopian tubes, continue to develop but are dysfunctional and infertile.
"Yet the father has the same Y chromosome and the same mutation as the daughter," Weiss pointed out. "And since he is a fertile male, we know that the switch must be poised right at its edge."
The team decided to measure the biochemical threshold of the SRY master switch.
"Our expectation was that we'd find that a factor of 100 or morea severe insult to the Y-encoded switchwas necessary to alter development," Weiss said. "But what we found was that the SRY threshold, as probed in father-daughter pairs, is only a factor of two."
Therefore, human males actually develop near the edge of sexual ambiguity. This means that, unlike the robust genetic programs which develop other essential processes like heart function, the SRY gene master switch is particularly vulnerable to change. It only takes a slight deviation from the normal process to dramatically alter fetal sexual development.
Given the importance of sexual reproduction to the survival of a species, why do human SRY genes function so close to the boundary of infertility? The idea of an unreliable master switch might appear paradoxical, but a growing body of research suggests that it might be an evolutionary necessity.
Extensive studies of gender-associated styles of childhood play and the acquisition of social competencies by Dr. S. Baron-Cohen and colleagues at Cambridge U
|Contact: Amanda Petrak|
Case Western Reserve University