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In addition, a eukaryotic gene does not code for a protein in one continuous stretch of DNA. Both exons and introns are "transcribed" into mRNA, but before it is transported to the ribosome, the primary mRNA transcript is edited.

This means that the gene's sequence is slightly different in the two individuals, and the gene is said to be polymorphic, "poly" meaning many and "morph" meaning shape or form.

Mutations can either be synonymous, in which the variation still results in a codon for the same amino acid or non-synonymous, in which the variation results in a codon for a different amino acid.

This process creates a stretch of hybrid DNA (bottom panel) in which the single strand of one duplex is paired with its complement from the other duplex.

However, evidence suggests that the mutations contribute to the progression of both diseases.

Because mitochondria have their own DNA, RNA, and ribosomes, this scenario is quite possible.

For example, mtDNA analysis has been used to trace the migration of people from Asia across the Bering Strait to North and South America.

It is still unclear what all the functions of introns are, but scientists believe that some serve as the site for recombination, the process by which progeny derive a combination of genes different from that of either parent, resulting in novel genes with new combinations of exons, the key to evolution.