Complex genetic models

The map of our chromosomes produced by the Human Genome Project has documented similarities and differences between individuals and has shown that people share approximately the same DNA with the exception of about 0.1% nucleotide bases [452]. These variations are called single nucleotide polymorphisms (SNPs) and occur when a single nucleotide (A, T, C, or G) in the genome sequence is altered. Some of these variants may be the cause of ‘monogenic diseases’ in which a single gene is responsible for causing the disease. This happens when the gene is defective so that a mutation of the DNA sequence of the gene determines a change in the associated protein. Our cells contain two copies of each chromosome and monogenic diseases are classified as ‘dominant’ or ‘recessive’ based on the number of copies of the mutated variants that are necessary for the disease to manifest. A dominant disease needs only one mutated variant to manifest while a recessive disease needs two mutated variants to manifest and one mutated variant makes a subject only a carrier of the disease. A carrier can transmit the mutated variant to the offspring, but not the disease and a recessive disease can only be