Some eggs receive the sperm, some do not. Therefore a gene in a male comes from his mother with the probability 1. On the other hand, the mother, which has two sets of chromosomes, apportions, through the crossover as usual, only half her genes to the offspring. The story differs, of course, for the fertilized eggs, but it never becomes boring. Fertilized eggs develop into females that not only share a father, but they de facto were conceived from identical sperm cells.
Evolution proceeds entirely without design, by mere chance.
This is to say that there exists a comprehensive and consistent explanation for every known facet of evolution based on the theory of probability and natural selection. Nonetheless, it is sometimes convenient to describe a phenomenon as if genes had a purpose and were developing survival strategies. Thus the language of game theory found its way into evolutionary biology. Besides altruistic attitude, evolution may lead to development of co-operation between different species or even individual genes. As described in Pinker , Partners in crime are held in separate cells, and the prosecutor offers each one a deal.