 quantitative trait loci (qtls) are stretches of dna containing or linked to the genes that underlie a quantitative trait. mapping regions of the genome that contain genes involved in specifying a quantitative trait is done using molecular tags such as aflp or, more commonly, snps. this is an early step in identifying and sequencing the actual genes underlying trait variation. quantitative traits refer to phenotypes (characteristics) that vary in degree and can be attributed to polygenic effects, i.e., product of two or more genes, and their environment. ==history== mendelian inheritance was rediscovered at the beginning of the 20th century, and as mendel's ideas spread geneticists began to connect mendel's rules of inheritance of single factors to darwinian evolution. for early geneticists, it was not immediately clear that the smooth variation in traits like body size was caused by the inheritance of single genetic factors. although darwin himself observed that inbred features of fancy pigeons were inherited in accordance with mendel's laws (although darwin didn't actually know about mendel's when he made the observation), it was not obvious that these features selected by fancy pigeon breeders can similarly explain quantitative variation in nature.http://www.literature.org/authors/darwin-charles/the-origin-of-species/ an early attempt by william ernest castle to unify the laws of mendelian inheritance with darwin's theory of speciation invoked the idea that species become distinct from one another as one species or the other acquires a novel mendelian factor.http://www.ncbi.nlm.nih.gov/pubmed/17752783 castle's conclusion was based on the observation that novel traits that could be studied in the lab and that show mendelian inheritance patterns reflect a large deviation from the wild type, and castle believed that acquisition of such features is the basis of "discontinuous variation" that characterizes speciation. interestingly, darwin discussed the inheritance of similar mutant features but did not invoke them as a requirement of speciation. instead darwin used the emergence of such features in breeding populations as evidence that mutation can occur at random within breeding populations, which is a central premise of his model of selection in nature. later in his career, castle would refine his model for speciation to allow for small variation to contribute to speciation over time. he also was able to demonstrate this point by selectively breeding a laboratory populations of rats to obtain a hooded phenotype over several generations.http://www.genetics.org/content/36/3/254.long castle's was perhaps the first attempt made in the scientific literature to direct evolution by artificial selection of a trait with continuous underlying variation, however the practice had previo
Classical_genetics Genetics Statistical_genetics