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In mammals, genetic information is carried by chromosomes. Each individual receives a version of each chromosome from each of its parents, forming a pair of homologous chromosomes. In cattle, there are 29 such pairs, in addition to the sex chromosomes. Interchromosomal rearrangements are the result of moving a chromosome segment to another non-homologous chromosome. The work published in this article presents a method developed by our research team to detect interchromosomal rearrangements using genotyping of millions of cattle. These genotypes tag the genome of each animal with several hundred markers per chromosome. By calculating the chance of a marker from one chromosome being systematically present at the same time as a marker from another chromosome in large siblings, it was possible to deduce the families in which a rearrangement was present. The presence of rearrangements was then validated using pre-existing methods.

This work is the first to take stock of the presence of these anomalies in the bovine species, and to quantify the occurrence of the phenomenon in insemination sires: out of 5571 siblings analyzed, 12 were found to carry one of these anomalies. Thanks to databases on bovine phenotypes in France, the study was also able to quantify the particularly deleterious effects of rearrangements on the fertility of carrier animals (halving the chances of successful mating) and their health (involving high mortality rates). These repercussions represent costly losses and unproductive periods for livestock farms, with serious consequences for the environment. In the final analysis, the distribution of a carrier bull costs production chains several hundred thousand euros.

Fortunately, there are means of prevention: karyotyping, monitoring and warning of fertility problems in insemination bulls and, last but not least, the earliest possible use of the method proposed by the researchers in this paper, as soon as the first calves are born from the same sire, to detect cases that have not yet been detected. Animals identified as carriers can then be excluded from renewal production to avoid spreading the anomaly through the generations.

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Description of illustration: Cattle genetic evaluations based on thousands of genetic markers are used by breeders for commercial and selection purposes. The knowledge gained from these evaluations can be used to make animals healthier and increasingly environmentally friendly. In this issue of Genome Research, an approach to detecting interchromosomal rearrangements in cattle by looking for abnormal patterns of linkage disequilibrium between markers located on different chromosomes is presented. This approach can be used to select animals that are healthier and will produce food for human consumption, reducing both losses and unproductive periods in breeding and the environmental footprint of agriculture. The cover shows a healthy cow crowned with a double helix of DNA. It is based on a cow photo taken by the author, modified manually and then using MidJourney (https://www.midjourney.com).
Cover No. 33 (6) Genome Research: https://genome.cshlp.org/content/33/6.cover-expansion