The November issue of the Register contained a Down to the Genes article focused on parentage testing. Here we republished a blog post with more in depth information on how the parentage analysis works and a hands-on example where readers can work through who is the correct sire.
By Jackie Atkins, Ph.D., Director of Science and Education
Originally published on the ASA Science blog, Mar 7, 2016.
Whether you use parent verification to confirm your breeding records, distinguish between AI and pasture bred calves, or to sort through progeny from a multi-sire pasture, parental validation is an important test for breeders. Roughly 10% of pedigrees reported across all breed associations are inaccurate. These are frequently honest mistakes but never-the-less, vital to fix and make informed breeding decisions. The theory behind parental validation is to determine if an animal could have inherited specific DNA markers from their reported parents. If the animal could not inherit a specific DNA Marker from the sire or dam, then the parent is excluded. With enough qualified markers, an animal is confirmed as the sire or dam of the offspring.
In the past, 15 microsatellite markers (also called Short Tandem Repeats or STRs) were used to confirm parentage but these were costly and not always reliable tests. Most new parental validation is completed with single nucleotide polymorphisms or SNPs (pronounced “snips”). The SNPs are locations in the DNA that vary from one animal to another. The SNP parentage test uses over 100 SNP markers to either qualify or exclude an individual as a parent. Each animal has two markers at each SNP location (one on each pair of chromosomes representing one from each parent).
Let’s work through a hypothetical example using SNPs for parental validation. We have test results for SNPs on a calf, two possible sires, and the likely dam. We look at each SNP and see if the calf could have inherited that SNP from the possible parents and also if the mating (the parents analyzed together) could have passed along those SNPs. The following diagram and table show four SNP markers for the individuals and the conclusions drawn.
Working through each SNP marker, we look if the calf could have inherited his DNA markers (genotype; options are A, T, C, or G) from the potential parents. At SNP 1, the calf has a genotype of A/T. This means the calf inherited an “A” from one parent and a “T” from the other parent. Both the Dam and Sire 1 are homozygous T and SNP1. This means they could only have passed along a T at this location. The potential dam and Sire 1 could not have conceived the calf together as neither could have passed along an “A” at this location. However, we can’t exclude either the dam or Sire 1 based on this information, we just know that they can’t be confirmed as the parents together.
Continuing through our analysis, we cannot exclude any of the parents with SNPs 2 and 3 because the pairing of both sires with the dam could pass on the calf’s SNP markers. Looking at SNP 4 is helpful. The calf has two “T”s at SNP 4, one from his dam and one from his sire. Sire 1 has two “A”s at SNP 4 so he could not have passed along a T at this location. Sire 1 is excluded based on this comparison. If Sire 1 is excluded, then the potential Dam and Sire 2 work for the remaining SNP locations. The parental validation at the lab uses the same logical progression as this simple example except it uses approximately 100 SNP markers for the analysis.