This type of natural selection is referred to as directional selection. In this type of natural selection, either one phenotype is preferred or the other; therefore, there is no equilibrium between the populations of the phenotypes that are present.
Examples of organisms that have undergone directional selection are black bears during the ice ages, African chichlid and sockey salmon.
I think that having an extra chromosome 21 is tolerable to the point that someone with this condition can survive to maturity because it does not replicate.
There should only be a pair of chromosome 21. In the event that there are 3 chromosome 21, the person that has this extra chromosome has Down syndrome. Changes in the number and structure of this chromosome may result to delayed development, intellectual disability, but not immediate death.
Glucose is broken down during fermentation anaerobically
The different forms a gene may have for a trait are called alleles. Blood type is an example of alleles. They determine the hereditary characteristics an offspring gains from their parents.