Re: If "Leadership" was a recessive gene ...
I think the main problem with this line of thought is that leadership is a fairly complex trait.
Traits are usually the product of natural selection, and therefore evolve according to the needs of the species within which they are found. We evolved the leadership gene because we are social animals, designed to work together in families, groups, societies, which in turn requires an adequate hierarchy, which in turn requires humans who are naturally predisposed to acting as leaders and humans who are naturally predisposed to acting as followers.
Now, genes as we know them are far more simple than we make them out to be. At its core, our DNA is merely a user manual, designed to create proteins, chemicals which can act as signals, hormones, or other such things. The hard to understand part of genetics is that, in order to overcome the inherent simplicity of the way our genes code information, they interact with one another and influence each other in a myriad of different ways.
The concepts of dominant and recessive are ones you will only find in things that are manipulated by a single gene, such as the shape of your ear, your eye color, etc. However, more complex traits, such as height, intelligence and leadership are defined by multiple genes, all of which play their part in modulating, stimulating or inhibiting the desired trait, which is why we have people who can never lead, people who could lead in certain circumstances, natural-born leaders, and even people who can eventually learn how to lead if given enough time.
So, to answer your question, turning leadership into a rarely expressed trait can have such a dramatic effect on humanity that we might not have been able to survive past the Ice Age, let alone reach the level of development and knowledge we have today, though this depends on how 'rare' you want this trait to become, and over what period of time. Slowly fazing the trait out over multiple generations might adapt human society into something different, while suddenly removing it in only one or two generations could lead to our society's destabilization and downfall.