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INBREEDING, LINE BREEDING and
MINIATURE HEREFORDS
An issue for
'closed herds', like the majority of miniature Herefords in
Australia, is the prevalence of inbreeding. High inbreeding
coefficients can reduce fertility (for example) and increase the
incidence of birth defects.
Inbreeding has always been of lively interest
to livestock breeders. Widely differing opinions exist between
breeders and even scientists on this subject. It is therefore not
uncommon to find many diverse breeding practices concerning
inbreeding in the livestock industry. Although some breeders claim
success with the use of inbreeding, it is generally accepted that
practicing inbreeding holds very few advantages and should at best
be considered risky if the large number of disadvantages are taken
into account.
Inbreeding defined
Inbreeding occurs when animals related to each
other are mated. An animal is inbred when both its parents have a
certain proportion of identical genes. The inbreeding coefficient is
defined as the probability that two identical genes (one from each
of the parents) is found at the same locus. The inbreeding
coefficient varies between 0 (0%) and 1 (100%) - a higher value
indicating that the parents of the animal concerned were more
closely related. This common heritage is expressed by a parameter
called the inbreeding coefficient, first proposed by Sewell Wright
in 1922.
The inbreeding coefficient is a function of
the number and location of the common ancestors in a pedigree. It is
not a function, except indirectly, of the inbreeding of the parents.
Thus, one can mate two highly inbred individuals who share little
common ancestry and produce a calf with a very low IC. (Because the
potential number of ancestors doubles every generation, eventually
you reach a point where the number of ancestors exceeds the number
of individuals alive at that time. Therefore, you are bound to find
some common ancestors if you go back far enough.) Conversely, it is
possible to mate two closely related individuals, both of which have
low ICs, and boost the IC substantially.
Unfortunately, in the average pedigree, there are a large number of
shared ancestors.
Therefore, the total inbreeding for a calf
cannot generally be calculated manually and appropriate software
must be used. Calculating inbreeding for only the first few
generations is not particularly useful. If there are more than one
or two common ancestors in four or five generation pedigree, the
inbreeding is probably already higher than desirable. Unfortunately,
having none is no guarantee that common ancestors will not occur in
abundance further back, and some pedigrees of this type still
achieve moderately high inbreeding coefficients. Neither can be
number of shared ancestors be used as a reliable guide, as the
inbreeding coefficient is very sensitive to when and where they
occur in a pedigree.
A low inbreeding coefficient indicates that a
calf has few common ancestors, thus minimising the chance of genetic
defects. Inbreeding and line-breeding really differ only in degree.
Line-breeding is less likely to cause harm than inbreeding.
Inbreeding is not for novices. Knowledge of genetics and the breed
is required for success. For good results, it must be well-planned
and breeders must be ready for whatever problems it presents.
Internet References
An excellent discussion paper can be found at
http://www.CompuPed.com/bell.htm , although this talks about dog
pedigrees, the ideas can be applied to miniature Herefords.
Another site is
http://www.inform.umd.edu/EdRes/Topic/AgrEnv/ndd/genetics/INBREEDING.html
where they talk of inbreeding in relation to dairy cattle.
Inbreeding: Its Meaning, Uses and Effects on
Farm Animals
http://muextension.missouri.edu/xplor/agguides/ansci/g02911.htm
Inbreeding in Swine
http://persephone.agcom.purdue.edu/~agcom/Pubs/NSIF/NSIF-FS4.html
Genetics of Reproduction: Considerations for
Sire Selection
http://muextension.missouri.edu/xplor/agguides/ansci/g02039.htm
Effects of Inbreeding and Heterozygosity on
Preweaning Traits in a Closed Population of Herefords Under
Selection
http://www.asas.org/abs/1998/may1303abs.html |
