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The aim of the study was twofold: firstly, to assess the relative importance of autosomes and the X chromosome on genetic variability in milk production and composition, fertility, udder health and cow stature; and secondly, to identify the X chromosome regions and genes involved in the genetic determinism of the traits analyzed.

The researchers studied over 230,000 cows from 6 French dairy breeds. They used data on the cows' milk production, resistance to udder infections, fertility and stature, as well as genotyping data on over 50,000 genetic variants spread across the entire genome, including 1,147 on the X chromosome. By comparing several thousand complete bovine genome sequences, they were able to predict the genotypes of each cow of the 6 breeds studied for almost 200,000 variants listed on the X chromosome. They then looked for associations between these variants and traits of interest in each breed, and to increase the power and resolution of their study, they combined the results obtained in the different breeds in a meta-analysis.

The X chromosome contributes an average of 3.5% of the genetic variation observed between individuals. In all the breeds studied, fertility and udder health, which are the most difficult to select for due to their lower heritability, are also the traits most influenced by this chromosome. The researchers identified specific regions and genes on the X chromosome associated with these traits. All these results demonstrate that the X chromosome plays a significant role in the processes of milk production, resistance to mammary infections, reproduction and growth in cows.

These findings hold great promise for the genetic improvement of dairy cattle. The integration of X chromosome genotyping data into the genomic prediction models used in breeding today should enable a gain in precision and therefore a more efficient selection of the best performing animals, with better milk production, increased resistance to disease, easier reproduction and adapted stature. What's more, as most of the genes linked to the X chromosome are shared between mammals, these results open up new prospects for a better understanding of its involvement in the genetic architecture of traits in domestic cows and cattle, as well as in other species.

Scientific contact

• Marie-Pierre Sanchez, UMR GABI