In its simplest term, inbreeding is the result of mating closely related individuals. Some examples of inbreeding can be seen in the figure below. Inbred offspring are more likely to inherit recent copies of the same allele from each parent (i.e., alleles that are identical by descent – derived from a common ancestor). One of the effects of inbreeding is that it causes an increase in the proportion of loci at which an individual is homozygous and a corresponding decrease in the proportion of loci at which an individual is heterozygous.
Prepotency in inbreds is a consequence of an increase in homozygosity caused by inbreeding. It is the ability of an individual’s progeny to perform similarly to that of its own performance. Prepotency is caused by inbred individuals having fewer heterozygous loci and thus having less genetic variation in gametes and consequently less variation among offspring. Prepotency is often used to introduce “uniformity”, but the importance of this is often over-emphasised by breeders. True prepotency (uniformity) is likely to only be observed in simply inherited traits (e.g., coat colour) or highly heritable polygenic traits. The reason for this is that when heritability is low the environmental effects influence performance to a much larger degree than the genetic effects, overwhelming the consequence of having “uniform” gametes. In general, inbred animals tend to be more sensitive to environmental stressors with respect to performance for polygenic traits, so although there might be genetic uniformity in inbreds they are not necessarily phenotypically more uniform in terms of performance.
At a farm level, one of the main concerns with inbreeding is inbreeding depression. Inbreeding depression refers to the reduction in average phenotypic performance as a result of inbreeding. The degree of effect and significance of inbreeding varies greatly across literature and appears to be largely dependent on population, breeding program design (closed, open or experimental populations), pedigree depth and completeness, and the alleles present (and their frequencies) within the populations. Some examples of the effects of inbreeding that have been observed are 2% increase in dystocia, 1% increase in still births, an increase of 8.8 days in calving interval and an increase of 2.5 days age at first calving (Gutierrez-Reinoso et al., 2022). In other studies, decreases in carcass weights of between 0.87 and 1.90kg per 1% increase in inbreeding were observed (McParland et al., 2007). The effect of inbreeding on scrotal circumferences observed by Pereira et al. (2016) was that a 0.07cm decrease in circumference was seen for every 1% increase in inbreeding. In the same study the average effect of the dam on calf weaning weight was -0.15kg lighter per 1% increase inbreeding and it was stipulated that this was due to inbred cows producing less milk. Extensive studies have been conducted on inbreeding depression in the dairy industry and some examples of the effects on production and conformation traits can be seen in the Table below (Gutierrez-Reinoso et al., 2022).
Genetic variation is one of the principal components when it comes to making genetic progress and care should be taken to maintain genetic diversity. The long-term maintenance of genetic diversity depends on the methods imposed to minimize the genetic relationships between selected individuals. For herds where inbreeding levels are relatively low and active steps are taken to manage the levels of inbreeding, the economic effects of inbreeding are likely to be low (Burrow, 1998; McParland et al., 2007). BREEDPLAN provides producers with tools such as mating predictor that allow producers to manage inbreeding within their herds. Mating predictor can be accessed through the following link (Simbra Mating Predictor (une.edu.au)).
For more information on inbreeding and inbreeding depression:
R. Bourdon, 2000. Understanding Animal Breeding. Pearson Education Limited.
H.M. Burrow, 1998. The effects of inbreeding on productive and adaptive traits and temperament of tropical beef cattle. Livestock production Science. 55, 227-243.
M.A. Gutierrez-Reinoso, P.M. Aponte, M. Garcia-Herreros, 2022. A review of inbreeding depression in dairy cattle: current status, emerging control strategies and future prospects. Journal of dairy Research 89, 3-12.
S. McParland, J.F. Kearney, D.E. MacHugh, D.P. Berry, (2007). Inbreeding effects on postweaning production traits, conformation, and calving performance in Irish beef cattle. Journal of Animal Sciences, 10.
K.A. Weigel, 2001. Controlling Inbreeding in Modern Breeding Programs. Journal of Dairy Science, 84, 177-184.