Final answer:
Genes such as the globin genes can occur close to one another in clusters. Each copy of a homologous pair of chromosomes originates from a different parent, resulting in genetic variation within individuals. Blood type is determined by the combination of two versions of the marker gene inherited.
Step-by-step explanation:
Most genes are single copy genes but some occur close to one another in clusters of related genes such as the globin genes. Each copy of a homologous pair of chromosomes originates from a different parent, so the genes themselves are not identical.
The variation in individuals within a species is due to the specific combination of genes inherited from both parents. For example, there are three gene sequences on the human chromosome that code for blood type - sequence A, sequence B, and sequence O. The blood type is determined by the combination of these two versions of the marker gene inherited.