Utilisation and conservation of farm animal genetic resources 147
Chapter 7. Genetic contributions and inbreeding
John Woolliams
1,2 1
Roslin Institute, Roslin Midlothian EH25 9PS, United Kingdom
2
Department of Animal and Aquacultural Sciences, Norwegian University of Life Science, P.O.Box 1432, Ås, Norway
Questions that will be answered in this chapter: How do we recognise inbreeding and how do we measure it?
What determines how fast inbreeding accumulates? What principles can be adopted to manage inbreeding?
How do these principles difer between breeding schemes primarily concerned with conservation and those primarily concerned with breed improvement?
How might rates of inbreeding be predicted? What general recommendations can be made for size of breeding schemes to achieve a
commonly accepted minimum Ne of 50?
Summary
his chapter examines how and why inbreeding accumulates in a population, and introduces the concept of genetic contribution as a simple means of describing the
rate of inbreeding ΔF. he relationship between ΔF and genetic contributions is used to derive the basic principles of managing a population to minimise ΔF. A further
relationship between the genetic contributions and the rate of gain in a selected trait is developed to illustrate how the relationships between long term genetic contributions
and rates of gain and inbreeding can lead to predictions on the optimum way to select for genetic improvement whilst managing the rate of loss of genetic variation. Approaches
to predicting ΔF are examined, and guidelines are given on ways to manage ΔF in practice depending on the sophistication of the breeding scheme. hese principles will
be developed further for practical implementation in chapter 8.
1. Inbreeding and inbreeding rate
Inbreeding was deined in chapter 3 and related aspects have been developed in previous chapters to identify resources for establishing conservation schemes and for uncovering
aspects of the history of the population. However the management of a genetic resource requires procedures for sustaining current diversity, including awareness of practices
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148 Utilisation and conservation of farm animal genetic resources
John Woolliams
that may unnecessarily increase inbreeding. herefore this chapter will discuss the concept of genetic contributions and how this simple concept can be used to inform
thinking.
Inbreeding coeicients are used extensively in the descriptions of populations, and those managing populations frequently ask ‘what is a safe level of inbreeding?’, particularly
when it is noted that inbreeding coeicients are all 0 in their population, with many no longer close to 0. However this is not an appropriate question for the following
reasons: 1. Inbreeding is inevitable. Inbreeding is 0 when an individual has two ancestors
in common and to avoid inbreeding, it is necessary to have 2 parents, 4 distinct grandparents, 8 distinct great-grandparents and so on. he number of distinct
ancestors required very quickly exceeds the population size in the past. 2. In a closed population inbreeding will increase and will eventually exceed any pre-
determined value of F. Lost alleles cannot be replaced without migration from another gene pool.
3. Each generation new mutations enter the population and these add to the population genetic variance, compensating in part for that variation lost by inbreeding.
Natural selection operates to remove potentially harmful inbreeding depression, providing ΔF is not too large so that the natural selection pressure can accumulate
chapter 8.
4. he current inbreeding coeicient of a population will depend on both 1 how
many generations back the base generation is and 2 how rapidly the inbreeding accumulates, measured by ΔF.
Item 4 indicates that since the choice of base generation is usually an arbitrary one and independent of the biological process of the inbreeding, then the important parameter
for management is ΔF. his perspective is supported by item 3.
Before looking at genetic theory, it is perhaps worthwhile examining some of the more obvious consequences of inbreeding in a population. he progress of inbreeding, in
particular when ΔF is high, is associated with inbreeding depression in which traits show a steady decline in performance as inbreeding progresses. hese traits are oten
those associated with itness, such as reproductive ability, and survival to breeding maturity, but many other physical traits such as growth rate or mature size will also show
depression. A summary of the impact of depression is given by Wiener
et al. 1994. his depression is linked to non-additive gene actions, which underlie the observation
of hybrid vigour, and it is the disappearance of heterozygosity in inbreeding chapter 3 that is responsible for depression since it reduces the opportunities for hybrid vigour to
be expressed. Traits associated with itness are widely considered to be the most sensitive
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