Evolution 101:
Evolution is not 'the best gene wins'. Hence why the analogy of red cubes and green cubes is erroneous. The central mechanism to evolution is mutation. Mutations occur periodically in DNA sequences- we've studied this, we know it occurs.
It is the same mechanism by which a pair of genetically healthy parents can have a genetically unhealthy child.
So you could in fact start with a scenario of purely 'good' genes (again, an oversimplification but one which is redundant to the point) and by the next generation have 'bad' genes introduced.
Mutations may be beneficial or not, depending on the nature of the genetic mutation, it's extent and the environmental factors involved- e.g. a genetic mutation which gives a creature slightly denser fur when that creature lives in a very cold environment will make that creature slightly more likely to survive and potentially pass this dense-fur gene on to the next generation; whereby over time there is the potential for this gene to propagate across the gene pool.
Similarly a 'bad' gene may still be passed on from one generation to the next if it does not prevent the organism which possesses the gene from procreating.
The key theme here is mutation, which can be positive or negative. Mutations happen, therefore there will always be a mixture of genes within an individual organism. The beauty of that being that if the environment suddenly changes, these so-called 'bad' genes may start proving useful and allow that organism a better chance at survival.
More information is available at your local library, Wikipedia, New Scientist, Google Reference Search and much more.
Dimitree did you honestly think you'd found the disproof to evolution which as escaped some of the brightest minds of the last 100 years?