<P> A typical protein - coding gene is first copied into RNA as an intermediate in the manufacture of the final protein product . In other cases, the RNA molecules are the actual functional products, as in the synthesis of ribosomal RNA and transfer RNA . Some RNAs known as ribozymes are capable of enzymatic function, and microRNA has a regulatory role . The DNA sequences from which such RNAs are transcribed are known as non-coding RNA genes . </P> <P> Some viruses store their entire genomes in the form of RNA, and contain no DNA at all . Because they use RNA to store genes, their cellular hosts may synthesize their proteins as soon as they are infected and without the delay in waiting for transcription . On the other hand, RNA retroviruses, such as HIV, require the reverse transcription of their genome from RNA into DNA before their proteins can be synthesized . RNA - mediated epigenetic inheritance has also been observed in plants and very rarely in animals . </P> <P> Organisms inherit their genes from their parents . Asexual organisms simply inherit a complete copy of their parent's genome . Sexual organisms have two copies of each chromosome because they inherit one complete set from each parent . </P> <P> According to Mendelian inheritance, variations in an organism's phenotype (observable physical and behavioral characteristics) are due in part to variations in its genotype (particular set of genes). Each gene specifies a particular trait with different sequence of a gene (alleles) giving rise to different phenotypes . Most eukaryotic organisms (such as the pea plants Mendel worked on) have two alleles for each trait, one inherited from each parent . </P>

Where do an organism’s genes come from