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These tests identify specific DNA sequences and nucleotide changes. They can be run at any age, often before the onset of clinical disease. Different types of genotypic tests reflect the research that identified them. Candidate gene studies or whole‐genome sequencing (WGS) usually identify specific disease‐causing mutations. In genome‐wide association studies (GWAS), groups of affected and normal individuals are compared at the DNA level. A SNP (pronounced snip) is a single nucleotide polymorphism – a single DNA letter (A, T, C, or G) that varies between individuals. GWAS utilizes gene chips that can evaluate hundreds of thousands of different SNPs. A SNP that is always present in a disease state but not in normal animals is considered a marker that lies close on the chromosome to a disease‐causing gene. In some instances, the SNP is a single nucleotide substitution that actually causes the disease.