A work's cast has more characters of one gender than the other.
I noticed that both Cast Full of Pretty Boys and Improbably Female Cast don't have a supertrope for some reason. Both tropes are getting misuse for general gender domination when they are more specifc than that.
This is when characters of the main cast are disproportionately of one gender than the other. In some works, it's to the point where virtually all characters are of one gender and people of the opposite gender don't seem to exist at all. The most common reason for this is that the work is designed to cater to one gender demographic and assumes that the audience will be more likely to related to characters that are of the same gender. Sometimes, this has the opposite effect of bringing in a Periphery Demographic of the opposite gender instead. Another reason is that the work is trying to provide fanservice to its intended male/female audience by making majority of the cast be the opposite (or same) gender. There is also the fact that a cast could be dominated by one gender because that's what's expected in the setting, like One Gender School. Supertrope to Cast Full of Pretty Boys and Improbably Female Cast. Related to The Smurfette Principle and Chromosome Casting. Contrast Gender Equal Ensemble.