Checking Parentage in Cattle with DNA
Scientists have long understood that each individual (with the exception of identical twins) is genetically unique. This applies to cattle as much as to people, dogs, horses and any other animal. Once DNA had been identified as the molecular basis of genes, this variability between individuals could be analysed in detail. Although the variation within genes is small between two individuals of the same species, DNA markers lying between the genes show much higher degrees of variability. Any two individuals will differ in many of these DNA markers and by analysing a set of the most variable markers we can build up a unique profile for an individual animal. The technology used for cattle is identical to that used for establishing human
paternity, the only difference being that the DNA markers used are derived from cattle, rather than human DNA.
DNA can be extracted from many different tissues. For cattle the simplest source is from hair roots, but DNA can be extracted from semen, blood, and also from muscle tissue when a dispute arises after slaughtering. Whatever the source, the first step in processing the sample is to extract the DNA. Because samples usually only contain very limited quantities of DNA, all the laboratory manipulations are carried out on a minute scale - the DNA from samples we routinely work with will typically be contained in a volume of 1/100,000 of a litre!
Quantities of DNA present in samples are far too small to measure successfully themselves, so we amplify up the DNA to a detectable level using the polymerase chain reaction - a technique that has revolutionised genetics since its development in the late nineteen eighties. This amplification specifically targets those segments of DNA which will give us a profile; coloured fluorescent dye is added to each fragment of amplified DNA at this stage so we can detect it.
After amplification the DNA is run on a Genetic Analyser (picture above) which separates out the DNA fragments. In practice, a number of DNA markers are amplified at the same time and identified from the Genetic Analyser by the length of the fragments and by the colour of the fluorescent dye on each one.
Detailed statistical analysis has demonstrated that a set of 11 different DNA markers is required for DNA profiling. This set has been standardised through the International Society for Animal Genetics. After conversion to this standard, DNA profiles can be exchanged between laboratories across the world.
For parentage testing, the DNA profile of the calf is compared with the profile of possible sires or dams. Each calf has two copies of each DNA marker, one inherited from the sire and one from the dam. For each of the eleven DNA markers, therefore, one of the copies in the calf must match one in the sire and the other copy match one in the dam. Any mis-matches will exclude the sire or dam (or both) from being the true parents. The data above shows results for one DNA marker for a typical paternity case. Each numbered peak represents one copy of the DNA marker in each of the three animals, two potential sires and the calf. The true father of the calf must be sire 1.The calf has copies of the marker designated 135 and 145. Only sire 1 has a matching copy, 135, and so sire 2 is excluded as the real father. The calf must have inherited the 145 copy of the marker from its dam.
