what are some of the essential ones?
Housekeeping genes
Gene
Essential genes include housekeeping genes (critical for basic cell functions) as well as genes that are expressed at different times in the organisms development or life cycle. 
dna that contains genes from more than one source is called UNANSWERABLE what does 'gene' means? A sequence of nucleotides in DNA or RNA that encodes the synthesis of a gene product, either RNA or protein what is its purpose? The transmission of genes to an organism's offspring is the basis of the inheritance of phenotypic trait.
wiki:15827936
Gene [SEP] Essential genes
Although the number is more difficult to measure in higher eukaryotes, mice and humans are estimated to have around 2000 essential genes (~10% of their genes). The synthetic organism, "Syn 3", has a minimal genome of 473 essential genes and quasi-essential genes (necessary for fast growth), although 149 have unknown function. Essential genes include housekeeping genes (critical for basic cell functions) as well as genes that are expressed at different times in the organisms development or life cycle. Housekeeping genes are used as experimental controls when analysing gene expression, since they are constitutively expressed at a relatively constant level.