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Connectedness
The use of BLUP is not a panacea for genetic evaluation. For an across-herd evaluation to be robust, animals in the different herds must either be tied together directly (through relatives) or indirectly through contemporaries. Such is particularly so when herds differ in their mean genetic merit, which is to be anticipated. Contemporaries are usually progeny of sires used widely within a breed. The closer these ties or connectedness, the more reliable the genetic evaluation.
The accuracy of an Expected Progeny Difference (EPD) itself is one item that affects genetic response. However, the level of precision (or error) when comparing EPD of two animals in different herds is equally important and often receives less attention. Since such between herd comparisons often determine which animals are actually chosen, accurate comparisons leading to the right choices have clear impact on genetic progress.
Connectedness affects the quality of selection decisions in two ways: the accuracy of the EPD itself and the precision of the comparison of EPD. As connectedness improves, error in estimating and comparing progeny differences falls. The variance, or the mean squared error, of prediction of the difference between EPD of different animals has therefore been proposed as an appropriate measure of connectedness.[1][2][3][4] Several connectedness statistics consequently have been based on the premise of prediction error variance.
- ↑ Foulley, J. L., E. Hanocq, and D. Boichard. 1992. A criterion for measuring the degree of connectedness in linear models of genetic evaluation. Genetics, Selection, Evolution 24:315-330.
- ↑ Kennedy, B. P., and D. Trus. 1993. Considerations on genetic connectedness between management units under an animal model. Journal of Animal Science 71:2341-2352. doi.org/10.2527/1993.7192341x
- ↑ Hanocq, E., D. Boichard, and J. L. Foulley. 1996. A simulation study of the effect of connectedness on genetic trend. Genetics, Selection, Evolution 28:67-82.
- ↑ Kuehn, L. A., R. M. Lewis, and D. R. Notter. 2007. Managing the risk of comparing estimated breeding values across flocks or herds through connectedness: a review and application. Genetics, Selection, Evolution 39:225–247. doi: 10.1051/gse:2007001 45.
