The terminology behind genomics
As the use of DNA technology continues to be incorporated into the beef cattle industry, there is a new world of terminology used in the discussion of DNA testing, information and genetic evaluation. This column will give you a road map on the terminology behind genomics by offering definitions of some of the more commonly used or referred to terms you may be hearing in reference to DNA technology within the beef cattle industry and genomic testing.
The following are some of the common terms used to describe the science of genetic prediction: Genome – The entire genetic makeup of an organism. The genome of an organism is stored in chromosomes located inside each cell.
DNA – (deoxyribonucleic acid) – Inside each cell found in living organisms is DNA. DNA is made up of pairs of four nucleotides abbreviated as “A”, “C”, “G” and “T”. DNA transmits genetic information between generations during reproduction and it also continually spells out the identity and the rate of assembly of proteins.
Proteins - are essential to the structure and function of plants and animals. The proteins produced by different alleles of genes may influence the phenotype or performance of the animal.
Chromosome – an organized structure of DNA and protein found in cells. Cattle have 30 chromosome pairs.
Gene – is a molecular unit of heredity of a living organism. A gene is a distinct sequence of DNA that contains all of the instructions for making proteins and other biologically important molecules. The DNA sequence that makes up a gene can differ between individuals and thus hold the information to build or maintain an organism’s cells and pass genetic traits to offspring.
Allele – The alternative DNA sequences of forms of a gene are called alleles. They can result in differences in the amount or type of protein being produced by that gene among different individual animals. This can affect the performance or appearance of animals that carry different alleles. Different alleles can result in different observable phenotypic traits such as pigmentation. Alleles can be recessive (meaning that an animal must inherit the same allele from both parents before there is an effect), additive (means carrying two copies of a particular allele produces double the effect of carrying one copy), or dominant (meaning that the presence of one allele is sufficient for expression of a phenotype).
Genetic Marker - a gene or DNA sequence with a known location on a chromosome that can be used to predict the genetic merit of an animal.
SNP – (single nucleotide polymorphism – referred to as a “snip”) a type of a genetic marker where alleles differ from each other by the sequence of only a single nucleotide base pair. SNP genetic tests focus on detecting precise single nucleotide base pair differences among the 3 billion nucleotide base pairs that make up the bovine genome. Commercial beef cattle tests continue to expand. Some tests measure 50,000 SNPs and future genetic tests may measure up to 700,000 SNPs.
Genotyping - the process of using laboratory methods to determine which DNA-marker alleles an individual animal carries, usually at one particular gene or location (locus) in the genome. The genotype identifies the marker alleles an animal carries.
Because an animal gets one allele of each gene from its sire, and one allele of each gene from its dam, it can only carry two alleles at any given marker locus or gene. If an animal gets the same marker allele from each parent, it is referred to as homozygous, or it may inherit different alleles from each parent, in which case it is referred to as heterozygous.
Complex traits – a trait influenced by many genes and also by the production environment. Many economically relevant traits for cattle production (birth weight, weaning weight, growth, reproduction, milk production, carcass quality, etc.) are complex traits.
These terms and additional genomic terms are defined in the sire selection manual research report on the National Beef Cattle Evaluation Consortium website (www.nbcec.org) and from presentations made at the 2011 Range Beef Cow Symposium. — Lynn Gordon, South Dakota State University