<P> "Orphan" genes, whose sequence shows no similarity to existing genes, are less common than gene duplicates . Estimates of the number of genes with no homologs outside humans range from 18 to 60 . Two primary sources of orphan protein - coding genes are gene duplication followed by extremely rapid sequence change, such that the original relationship is undetectable by sequence comparisons, and de novo conversion of a previously non-coding sequence into a protein - coding gene . De novo genes are typically shorter and simpler in structure than most eukaryotic genes, with few if any introns . Over long evolutionary time periods, de novo gene birth may be responsible for a significant fraction of taxonomically - restricted gene families . </P> <P> Horizontal gene transfer refers to the transfer of genetic material through a mechanism other than reproduction . This mechanism is a common source of new genes in prokaryotes, sometimes thought to contribute more to genetic variation than gene duplication . It is a common means of spreading antibiotic resistance, virulence, and adaptive metabolic functions . Although horizontal gene transfer is rare in eukaryotes, likely examples have been identified of protist and alga genomes containing genes of bacterial origin . </P> <P> The genome is the total genetic material of an organism and includes both the genes and non-coding sequences . </P> <P> The genome size, and the number of genes it encodes varies widely between organisms . The smallest genomes occur in viruses, and viroids (which act as a single non-coding RNA gene). Conversely, plants can have extremely large genomes, with rice containing> 46,000 protein - coding genes . The total number of protein - coding genes (the Earth's proteome) is estimated to be 5 million sequences . </P>

When are gene products from these genes made and where are they located