What are the disadvantages to asexual reproduction and why is this disadvantage not true for sexual reproduction?

The disadvantage is no genetic variation, this disadvantage is not true for sexual reproduction because different chromosomes from two different individuals are combined so genetic variation is evident in sexual reproduction.

Disadvantages: Longer Negative Mutations and less Genetic Diversity

Explanation:

Negative Mutations live longer in the genomes that are in existence as the result of asexual reproduction. The main reason behind these longer living genetic mutations is the less variations producing capability in the case of asexual type of reproduction. These negative mutation are very less in genomes that are formed by crossing over positive mutations induced genome.

Genetic Diversity is very less in case of asexual type of reproduction. This is because there is no shuffling of genes in chromosomes takes place as compared to the sexual reproduction. This crossing over shuffling is very important in producing variations that will ultimately produced a good genetic diversity.