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The Genom of Homo sapiens.pdf

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186 GEORGES AND ANDERSSONin humans (Milan et al. 2000). Other examples abound:<strong>The</strong> differences in skeletal muscle mass and fat depositionamong pig and poultry breeds as described in this papercan be viewed as models <strong>of</strong> mild obesity; understandingthe differences in calcium metabolism between junglefowl and leghorn chickens linked to egg production couldreveal pathways relevant to osteoporosis; cystic ovariandisease is a very common complex inherited disease indairy cattle and potentially a good model for polycysticovarian syndrome, one <strong>of</strong> the most common endocrinedisorders <strong>of</strong> women.Contrary to previous considerations (Nadeau andFrankel 2000), we believe that QTL mapping and subsequentQTN identification have the potential to make asignificant contribution to narrow the “phenotype gap”;i.e., the lack <strong>of</strong> functional information from mutation-inducedphenotypes for most mammalian genes. <strong>The</strong> detection<strong>of</strong> the QTN underlying the SSC2 QTL illustrates thisvividly. <strong>The</strong> Q to q substitution effect corresponds to adifference <strong>of</strong> 2–3% in muscle mass, which would havebeen virtually impossible to detect in a phenotype-drivenmutagenesis screen. Yet this mutation accounts for 25%<strong>of</strong> the phenotypic difference in the F 2 generation, and itsidentification revealed a novel cis-acting regulatory elementin IGF2, a gene that has been extensively studiedusing standard molecular biology.<strong>The</strong> genome sequences <strong>of</strong> a number <strong>of</strong> livestockspecies are expected to become available in the not toodistant future. <strong>The</strong> poultry genome should be completed,at least at eightfold coverage, by the end <strong>of</strong> 2003(http://genomewustl.edu/projects/chicken/), and the sequencing<strong>of</strong> the bovine genome should be initiated beforethe end <strong>of</strong> the year. A onefold coverage <strong>of</strong> the porcinegenome will be generated by the end <strong>of</strong> 2003 (M. Fredholm,pers. comm.), and more extensive sequencing <strong>of</strong>the pig genome will hopefully commence soon after thecompletion <strong>of</strong> the poultry and bovine genomes.So far, the motivation to sequence the genomes <strong>of</strong> livestockspecies has mainly come from the realization thatcomparison <strong>of</strong> genome sequences from evolutionarily diversespecies is a powerful approach to identify functionallyimportant genetic elements (see, e.g., Collins et al.2003). Having the sequence <strong>of</strong> the livestock genomes athand will also immensely facilitate the identification <strong>of</strong>additional QTNs in these species in which—contrary tothe status in the human—large numbers <strong>of</strong> very convincingQTLs have already been mapped. <strong>The</strong> potential value<strong>of</strong> understanding the molecular architecture <strong>of</strong> QTLs segregatingin livestock populations not only for agriculturebut, equally importantly, for fundamental biology and thebiomedical science, is an additional reason to ensure thatlivestock genomes are rapidly and completely sequenced.ACKNOWLEDGMENTSResults described in this paper were made possiblethanks to the financial support <strong>of</strong> the Belgian and WalloonMinistries <strong>of</strong> Agriculture, the EU, Cr-Delta (Arnhem,<strong>The</strong> Netherlands), LIC (Hamilton, New Zealand),VLB (Auckland, New Zealand), Gentec (Buggenhout,Belgium), and the Swedish Research Council for Environment,Agricultural Sciences and Spatial Planning. Weare grateful to Dr. Fredholm for critically reviewing thismanuscript.REFERENCESAmarger V., Nguyen M., Van Laere A.S., Nezer C., Georges M.,Andersson L. 2002. Comparative sequence analysis <strong>of</strong> theINS-IGF2-H19 gene cluster in pigs. Mamm. <strong>Genom</strong>e 13: 388.Andersson L. 2001. Genetic dissection <strong>of</strong> phenotypic diversityin farm animals. Nat. Rev. Genet. 2:130.Andersson L., Haley C.S., Ellegren H., Knott S.A., JohanssonM., Andersson K., Andersson-Eklund L., Edfors-Lilja I.,Fredholm M., and Hansson I., et al. 1994. Genetic mapping <strong>of</strong>quantitative trait loci for growth and fatness in pigs. Science263:1771.Andersson-Eklund L., Marklund L., Lundstrom K., Haley C.S.,Andersson K., Hansson I., Moller M., and Andersson L. 1998.Mapping quantitative trait loci for carcass and meat qualitytraits in a wild boar x Large White intercross. J. Anim. 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