This position is called locus . Particular set of genes in genome is called genotype . The genotype is with later development after birth base for the organism's phenotype , its physical and mental characteristics, such as eye color, intelligence etc. Genes from parents form in some way the whole new chromosome. The new created offspring can then be mutated.
Mutation means, that the elements of DNA are a bit changed.
This changes are mainly caused by errors in copying genes from parents. The fitness of an organism is measured by success of the organism in its life.