and between the muscles, inside organs, and layered under the skin.” Other production traits are meat tenderness and palatability, as well as milk production.

Kappes pointed out that in many cases scientists already have found the general location on a chromosome for a gene that expresses a particular trait in an animal. The technical term for the general location of a gene, which affects a particular trait that is measured on a quantitative scale, is a “quantitative trait locus,” or QTL. “We have in excess of 30 QTLs in cattle for these different traits,” Kappes said, plus a similar number in pigs and a dozen or so in chickens. Once a particular animal genome is sequenced, it might be possible to determine which gene or genes affect a trait, and thus give breeders the information they need to enhance the production traits of that animal.

SEQUENCING FOR ENHANCED BASIC SCIENTIFIC UNDERSTANDING

Besides the agricultural benefits, genomic sequencing of domestic animals will be important in a number of areas of basic science, particularly in understanding the evolutionary relationships between species. “Nothing in biology really makes sense except in light of evolution,” commented Stephen O’Brien, chief of the Laboratory of Genomic Diversity and head of the Section of Genetics at the National Cancer Institute (NCI). The genome of each species is the end result of millions of years of mutation and natural selection. The genome of every mammal alive today, for instance, can be traced back to the genome of an ancestral mammal that lived some 200 million years ago, and the genomes of the different species provide a record of how the descendants of that proto-mammal gradually diverged into many different forms, as well as a guide to how today’s mammals are related to one another.

When we explore the evolution of a living species, we assume that its presence here today is the result of that species successfully adapting and negotiating the myriad of ecologic and environmental challenges over time, O’Brien explained. “Nestled in the genomes of living species are the historic footprints of the adaptive events that led them to where they are today.” In other words, comparing the genomes of various mammals alive today might be the best option for understanding how the species evolved as they did.

Several other evolutionary questions can be addressed by sequencing a variety of mammals, O’Brien said. “We don’t know which of the genes make us human, as opposed to apes or as opposed to non-ape primates, or as opposed to other orders of mammals.” Only by sequencing the genomes of other animals and comparing them gene by gene with the human genome will it be possible to answer this fundamental question. Some preliminary comparisons between humans and other mammals already have been made, said Harris Lewin of the



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