<P> Heritable traits that result from infection with foreign particles have also been identified in Drosophila . Wild type flies normally full recover after being anesthetized with carbon dioxide . Certain lines of flies have been identified that die off after exposure to the compound . This carbon dioxide sensitivity is passed down from mothers to their progeny . This sensitivity is due to infection with σ (Sigma) virus, a rhabdovirus only capable of infecting Drosophila . </P> <P> Although this process is usually associated with viruses, recent research has shown that the Wolbachia bacterium is also capable of inserting its genome into that of its host . </P> <P> Genomic imprinting represents yet another example of non-Mendelian inheritance . Just as in conventional inheritance, genes for a given trait are passed down to progeny from both parents . However, these genes are epigenetically marked before transmission, altering their levels of expression . These imprints are created before gamete formation and are erased during the creation of germ line cells . Therefore, a new pattern of imprinting can be made with each generation . </P> <P> Genes are imprinted differently depending on the parental origin of the chromosome that contains them . In mice, the insulin - like growth factor 2 gene undergoes imprinting . The protein encoded by this gene helps to regulate body size . Mice that possess two functional copies of this gene are larger than those with two mutant copies . The size of mice that are heterozygous at this locus depends on the parent from which the wild type allele came . If the functional allele originated from the mother, the offspring will exhibit dwarfism, whereas a paternal allele will generate a normal sized mouse . This is because the maternal Igf2 gene is imprinted . Imprinting results in the inactivation of the Igf2 gene on the chromosome passed down by the mother . </P>

When do inheritance patterns deviate from simple mendelian genetics