Genetic gain & Inbreeding

Selection on polygenic traits reduce the genetic variance considerably more due to the Bulmer effect than typically assumed

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Abstract

Selection reduces the genetic variance. A large part of this reduction is a result of a transient phenomenon known as the "Bulmer effect", which originates from the creation of negative covariances between loci. By assuming the infinitesimal model, the Bulmer effect is usually accounted for when estimating breeding values and genetic variance components. However, there is increasing evidence that breeding goal traits are influenced by very many loci that are not segregating independently because the number of chromosomes is limited. We aim to investigate the impact of this deviation from the infinitesimal model on the genetic variance under selection. Our results show that the reduction in genetic variance due to the Bulmer effect can be considerably larger than typically believed, and that it takes much longer for the genetic variance to return to its original (genic) value. Previously derived theory shows that the reduction in genetic variance is a function of the harmonic mean of the recombination rate (rH) between loci. The rH is 0.5 when all loci are on separate chromosomes, resulting in a drop of ~21% of the genetic variance for mass selection on a trait with a heritability of 0.4 and using a selection intensity of 20%. When the genome contains 30 chromosomes with a genome size of 30 Morgan, the rH is ~0.33 when 30,000 loci are randomly spread along the genome, resulting in a drop of ~28%. We confirmed those predictions by simulating a population with 10 chromosomes of 1 Morgan each under mass selection for 50 generations for a single trait with a heritability of 0.4 affected by 2000 additive loci, followed by 500 additional generations of random selection. After 50 generations of selection, the genetic variance was ~34% lower than the genic variance which represents the relative reduction due to the Bulmer effect. After selection was stopped, the genetic variance substantially increased in the first generations after selection due to the breakdown of negative covariances between chromosomes. However, about 150 to 250 generations of random selection were needed before all negative covariances were broken down and the genetic variance reached the same value as the genic variance. When each causal locus was simulated on a separate chromosome to resemble the infinitesimal model, the relative reduction due to the Bulmer effect was only ~17% and the genetic and genic variance became similar within seven generations of random selection. Altogether, those results show that the impact of the Bulmer effect can be much larger than predicted by the infinitesimal model. Therefore, the use of infinitesimal model assumptions, as for example done in the Animal Model, can create bias when estimating breeding values and genetic variance components.

Keywords: 2026

How to Cite:

Wientjes, Y., Calus, M. & Bijma, P., (2026) “Selection on polygenic traits reduce the genetic variance considerably more due to the Bulmer effect than typically assumed”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2255379. doi: https://doi.org/10.31274/wcgalp.23387

Rights: 1