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Both the number of base pairs and the number of genes vary widely from one species to another, and there is little connection between the two (an observation known as the C-value paradox ). At present, the highest known number of genes is around 60,000, for the protozoan causing trichomoniasis (see List of sequenced eukaryotic genomes ), almost three times as many as in the human genome .

Note that a genome does not capture the genetic diversity or the genetic polymorphism of a species. For example, the human genome sequence in principle could be determined from just half the information on the DNA of one cell from one individual. To learn what variations in genetic information underlie particular traits or diseases requires comparisons across individuals. This point explains the common usage of "genome" (which parallels a common usage of "gene") to refer not to the information in any particular DNA sequence, but to a whole family of sequences that share a biological context.

Although this concept may seem counter intuitive, it is the same concept that says there is no particular shape that is the shape of a cheetah . Cheetahs vary, and so do the sequences of their genomes. Yet both the individual animals and their sequences share commonalities, so one can learn something about cheetahs and "cheetah-ness" from a single example of either.

Comparison of different genome sizes

Organism Genome size ( base pairs ) Note
Virus , Bacteriophage MS2 3,569 First sequenced RNA-genome [3]
Virus , SV40 5,224 [4]
Virus , Phage Φ-X174; 5,386 First sequenced DNA-genome [5]
Virus , Phage λ 48,502
Bacterium , Haemophilus influenzae 1,830,000 First genome of living organism, July 1995 [6]
Bacterium , Carsonella ruddii 160,000 Smallest non-viral genome. [7]
Bacterium , Buchnera aphidicola 600,000
Bacterium , Wigglesworthia glossinidia 700,000
Bacterium , Escherichia coli 4,600,000 [8]
Amoeba , Amoeba dubia 670,000,000,000 Largest known genome. [9]
Plant , Arabidopsis thaliana 157,000,000 First plant genome sequenced, Dec 2000. [10]
Plant , Genlisea margaretae 63,400,000 Smallest recorded flowering plant genome, 2006. [10]
Plant , Fritillaria assyrica 130,000,000,000
Plant , Populus trichocarpa 480,000,000 First tree genome, Sept 2006
moss , Physcomitrella patens 480,000,000 First genome of a bryophyte , January 2008 [11]
Yeast , Saccharomyces cerevisiae 12,100,000 [12]
Fungus , Aspergillus nidulans 30,000,000
Nematode , Caenorhabditis elegans 98,000,000 First multicellular animal genome, December 1998 [13]
Insect , Drosophila melanogaster aka Fruit Fly 130,000,000 [14]
Insect , Bombyx mori aka Silk Moth 530,000,000
Insect , Apis mellifera aka Honey Bee 1,770,000,000
Fish , Tetraodon nigroviridis , type of Puffer fish 385,000,000 Smallest vertebrate genome known
Mammal , Homo sapiens 3,200,000,000
Fish , Protopterus aethiopicus aka Marbled lungfish 130,000,000,000 Largest vertebrate genome known

Lecture 13. living organisms share common genes

All organisms store genetic information in the same molecules - DNA or RNA. Written in the genetic code of these molecules is compelling evidence of the shared ancestry of all living things. Evolution of higher life forms requires the development of new genes to support different body plans and types of nutrition. Even so, complex organisms retain many genes that govern core metabolic functions carried over from their primitive past.

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Source:  OpenStax, Genetics. OpenStax CNX. Jul 29, 2009 Download for free at http://cnx.org/content/col10782/1.1
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