Managing Inbreeding within the Suffolk Breed
Shane Conway Signet Breeding Services, January 2013
Summary
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Inbreeding issues can only be fully understood when pedigrees are
known and accurately recorded.
Breeding decisions have to be a balance between making faster rates
of genetic gain and minimising rates of inbreeding
Some inbreeding is inevitable in most populations – whether the flock
is involved in performance recording or not. Breeders should be aware
of the impact that inbreeding can have on their breeding programme.
The level of inbreeding within a flock can be measured
Software can be developed to predict future levels of inbreeding
between specific matings. This would enable breeders to minimise the
risks associated with inbreeding.
What is inbreeding?
Inbreeding is the practice of mating two genetically related animals. To a degree this
is inevitable within any long-term selection programme involving a closed population.
Breeders will sometimes deliberately inbreed specific bloodlines to fix certain
characteristics in the flock, increasing the frequency of favourable genes – or more
rarely to expose recessive genes.
Inbreeding may have a place in a breeding strategy as in some instances it will bring
a number of favourable genes together. However, it will also tend to increase the
number of recessive or deleterious genes being expressed – leading to a reduction in
fitness and productivity; this is referred to as “inbreeding depression”.
The challenge to avoid inbreeding
To make fast genetic progress in any particular attribute a small number of rams and
ewes that excel in that attribute tend to be mated together. If this approach is taken to
an extreme, it can quickly result in an increase in the level of inbreeding within the
flock.
Modern livestock breeding programmes can be susceptible to increases in inbreeding
due to the widespread use of AI & ET, fast generation turnover, selective use of
specific family lines and the tendency for a relatively small number of different sire
families to dominate within certain breeds. Increases in inbreeding are observed in
both recording and non-recording flocks.
Breeders need to strike a balance to optimise rates of genetic gain, whilst controlling
increases in levels of inbreeding.
Calculating Levels of Inbreeding
The level of inbreeding is calculated as the probability of two alleles being identical by
descent. This value is called the "inbreeding coefficient".
Typical inbreeding percentages are as follows, assuming no previous inbreeding
between any parents
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Father/daughter, mother/son or brother/sister → 25%
Grandfather/granddaughter or grandmother/grandson → 12.5%
Half-brother/half-sister → 12.5%
Uncle/niece or aunt/nephew → 12.5%
Great-grandfather/great-granddaughter or great-grandmother/great-grandson
→ 6.25%
Should breeders completely avoid inbreeding?
Complete avoidance of inbreeding is almost impossible and a balance has to be
struck between the genetic superiority of a specific ram and the level of inbreeding he
creates in the future flock. Low levels of inbreeding (<6.25%) are often considered an
acceptable compromise. Levels higher than that should be avoided.
The best solution to the inbreeding challenge is to measure the level of inbreeding
between individuals and make recommendations based on this information.
What can the Suffolk breed do?
There are two things the Suffolk breed can do.
1). Calculate and publish the inbreeding co-efficient of every animal in the database.
This would enable: Inbreeding to be monitored at a breed level
 Inbreeding to be monitored and reported at a flock level
2). Consider providing web based software to breeders that enables them to work out
inbreeding co-efficients between specific individuals.
This would enable Suffolk breeders to work out how closely related future matings are
likely to be and manage the inbreeding risk accordingly
Summary
Breeders shouldn’t stop making good use of the reproductive and genetic tools
available to them – but they can take steps to minimise increases in inbreeding now
and in the future.
Appendix 1. Inbreeding in Suffolk Sheep
Signet has looked at the rates of inbreeding within the Suffolk breed.
This work was based on calculations undertaken by SAC based on Suffolk Run 2
2012. Signet extracted data for male lambs only (to avoid any female recipient ewe
bias). The chart below shows the level of inbreeding in Suffolk lambs born since 2007
– and their parents in each given year.
While rates of inbreeding within the breed appear relatively low, they have increased
in the last 2 years – in part due to the use of sires with slightly higher levels of
inbreeding.
The chart shows a change in the rates of inbreeding, but care should be taken in
determining the significance of this difference.
Signet has chatted to Marco Winters, DairyCo Geneticist about this subject over the
summer and Marco made the following quick observations.
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Inbreeding coefficients are highly dependent on depth of pedigree, and therefore the
level can sometime be under estimated. Direction and speed of change may therefore
be more relevant to the sheep industry.
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The 2% inbreeding co-efficient seen here (assuming it is calculated on deep pedigrees
>5 generations) is perfectly acceptable. Anything less than 6% is nothing to worry
about in the medium term.
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Speed of change is many respects is more important; 1% over 5 years would be fairly
rapid. In the case of Suffolk’s the change is about 0.5% over 5 years (very similar to
the dairy industry 0.1% per annum), which is not unexpected and perfectly
manageable
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Whilst these rates of inbreeding are not a major concern at the moment - at this rate
you do want to monitor it and talk to breeders about this issue.
Appendix 2.
Breakdown of the Inbreeding Co-Efficients for the 2011 Suffolk Lamb Crop
This table shows the breakdown of inbreeding co-efficients for the Suffolk population.
While the vast majority have very low levels of inbreeding – it is interesting to note
that about 6% of the population already have inbreeding co-efficients above the 7%
threshold usually set as a sensible upper limit for inbreeding.
Inbreeding Co-Efficient
(2011 crop)
0
under 1
under 2
under 3
under 4
under 5
under 6
under 7
under 8
under 9
under 10
under 11
under 12
12 to 25
over 25
Number
of
Records
354
8543
1782
825
592
337
188
186
113
90
46
29
20
174
107
Proportion of
population
2.64%
63.82%
13.31%
6.16%
4.42%
2.52%
1.40%
1.39%
0.84%
0.67%
0.34%
0.22%
0.15%
1.30%
0.80%
Appendix 3.
Inbreeding and influence
This chart shows the inbreeding co-efficient of sires used to produce the 2011
lamb crop plotted against progeny number. The chart shows a number of sires
with inbreeding co-efficients over 7% being used to produce lambs.
This isn’t a problem providing they are being used as an outcross – but it is
indicative of a gradual increase in inbreeding levels in the population.