Spreading Altruism By Jumping On A Live Grenade

On the comments thread of Why Evolution Is True, Prof. Jerry Coyne uses a fun example to explain the concept of inclusive fitness to a reader:

josh ozersky-
Can someone please explain what inclusive fitness is?

whyevolutionistrue-

Basically, it’s calculated for a gene that does something like affect behavior, and it’s the relative fitness of that form of a gene compared to other forms that don’t have that behavior, counting the copies of that gene in related individuals. For example, a gene that codes for this behavior: “commit suicide (e.g. by falling on a grenade) if you can save more than two brothers or sisters by doing so” has a higher inclusive fitness than a gene that says “don’t fall on the grenade”, because if you die you lose one copy of that gene but save 1.5 others (you’re 50% related to those three siblings), while if you don’t fall on the grenade (and run away), you save your own copy but lose 1.5 others. The gene for the “altruisitc” behavior will spread because genes for it have a higher INCLUSIVE FITNESS than the other form of the gene.

Is that clear?

The discussion is with reference to Prof. Coyne's post on E.O. Wilson's book Sociobiology and his recent profile by Jenny Schuessler in the New York Times.