I'm a layman in genetics, but I believe the effect you described is what is known as Fisher's principle. Basically, when using sexual reproduction, it _would_

I'm a layman in genetics, but I believe the effect you described is what is known as Fisher's principle. Basically, when using sexual reproduction, it _would_ be highly effective for the species to have many more females than males, since females are the bottleneck of population growth. However, if the sex ratio is skewed then any traits which increases the likelihood of male offspring will enjoy a reproductive advantage (because the male offspring will itself have more offspring than the females), spread through the next generation, and reduce the sex ratio, until it reaches the balance of 50/50. This applies whether the trait is dominant or regressive, etc. So in this scenario, as the tuskless mutation spreads, the number of males decreases and the genetic advantage of having tusks grows, until it balances out the genetic disadvantage of losing females to poachers.