SummaryThis paper reviews how mammalian genomes are utilized in modern genetics for the detection of genes and polymorphisms (mutations) within domesticated animal (mostly livestock) genomes that are linked to traits of economic importance to humans. Examples are given of how genetic analysis allows to determine key genes associated with the quality and quantity of milk in cattle and key genes for meat production. Various questions are reviewed, such as how contemporary methods of genome sequencing allow to maximise the effective detection of genic and regulatory DNA polymorphisms within the genomes of major domesticated mammals (cattle, sheep and pigs) and the history of their formation from the standpoint of genetics.
The Structure of the GenomeWithin mammalian genomes, up to 40% of the DNA is comprised of simple repeats, DNAand retro-transposons, SINE and LINE elements, most of which do not encode for proteins (Fig. 1). Out of 30% of DNA that belongs to genes, only 2% is coding DNA (exons).Heterochromatin DNA, repetitive and unique noncoding DNA sequences comprise a further 30% of the genome. Variation within DNA sequences in the genome as a result of errors during replication is the basis for natural (during evolution) or artificial (animal breeding) selection.
Genes and mutations affecting economically-important traits in domesticated animalsThe principle of "candidate gene" search is presented in Fig. 2. Through the use of linkage mapping techniques, it can be determined that a specific chromosomal locus controls an economically important trait in livestock. Using the methods of comparative mapping, one can find the homeologous regions of human or mouse chromosomes corresponding to this locus. The genes that are present in human or mouse heomologous intervals are screened for ones performing a role similar to the one which is studied in the livestock animal. Gene(s) that control similar traits in a human or mouse, are then cloned and sequenced in the animal. The mutations (e.g. point mutations) are determined in these genes in livestock animal populations, and an associative analysis is conducted, which reveals the relationship (or lack thereof) between the animal's phenotype and specific nucleotide substitution(s). If an association of this sort is observed, then the gene that influences a particular trait in the animal's genome was identified correctly. Obviously, this approach has a number of drawbacks, namely: a) it "works" only for genes with similar In cases where the trait is controlled by multiple genes, the situation becomes more complicated, since often the contribution of each of these genes to the phenotype is quite small and often it is not possible to find one or two major genes/mutations that would significantly change phenotype in the desired direction. As shown in Table 1 As shown above, methods of genetic analysis allow for the identification of individual genes and mutations that have a desired effect on a number of important traits in domesticated animals. However, the traits are often...