  This paper develops simplified mathematical models describing the
mutation-selection balance for the asexual and sexual replication pathways in
{\it Saccharomyces cerevisiae}. We assume diploid genomes consisting of two
chromosomes, and we assume that each chromosome is functional if and only if
its base sequence is identical to some master sequence. The growth and
replication of the yeast cells is modeled as a first-order process, with
first-order growth rate constants that are determined by whether a given genome
consists of zero, one, or two functional chromosomes. In the asexual pathway,
we assume that a given diploid cell divides into two diploids. In the sexual
pathway, we assume that a given diploid cell divides into two diploids, each of
which then divide into two haploids. The resulting four haploids enter a
haploid pool, where they grow and replicate until they meet another haploid
with which to fuse. When the cost for sex is low, we find that the selective
mating strategy leads to the highest mean fitness of the population, when
compared to all of the other strategies. We also show that, at low to
intermediate replication fidelities, sexual replication with random mating has
a higher mean fitness than asexual replication, as long as the cost for sex is
low. This is consistent with previous work suggesting that sexual replication
is advantageous at high population densities, low replication rates, and
intermediate replication fidelities. The results of this paper also suggest
that {\it S. cerevisiae} switches from asexual to sexual replication when
stressed, because stressful growth conditions provide an opportunity for the
yeast to clear out deleterious mutations from their genomes.
