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The Analysis of Genetic Diversity in
Cattle to Preserve Future Breeding Options
Genetic Erosion of Cattle Populations
The genetic diversity available
for selection will affect the success of future cattle breeding
strategies. Selective breeding programmes result in genetic
erosion and inevitably a reduction in the genetic base from
which to select desired phenotypes in the future. Such genetic
erosion can occur for many different reasons:
- Extensive use of artificial insemination
has reduced the number of breeding sires and may result
in high levels of inbreeding and a restriction of the gene
pool. In many European countries, the leading sire may account
for up to 25% of pedigree inseminations in some breeds.
Consequently there is also inadvertent selection of genetic
defects.
- Export of semen or embryos exclusively
from breed leaders giving a restricted genetic pool in exotic
locations.
- Use of a restricted number of 'improved'
breeds, in particular the preferred use of Holstein/Friesian
cattle for dairy production and as a by-product for beef,
has led to the reduction of other breeds, with traditional
breeds being replaced and potentially lost. The traditional
breeds are often adapted to local conditions through many
years of selection - for example, breeds that are able to
thrive at altitude on alpine summer pastures, or in arid
conditions in the southern EU states. Such breeds may also
be better adapted to locally produced forage, or be more
resistant to geographically localised pathogens or pests.
- Commitment to high input farming
forcing the use of high yielding breeds in areas where this
is unsuitable. The selection for high yielding cattle does
not necessarily consider efficiency of production. In simplest
terms this means that cattle cannot be sustained on locally
produced fodder and farmers become committed to intensive
farming practices and the use of high energy, high cost
compounded feed which often has to be imported. To be viable,
the size of individual herds has also increased and this
has associated pollution problems.
Measuring Genetic Diversity
Blood Groups
Early work to measure genetic diversity
used blood groups to show differences between breeds and
the diversity present. Unfortunately, the number of loci
available are limited, with only the B system being sufficiently
polymorphic to be really useful. However, since there is
a wealth of information available from such typing, this
information can be used to estimate changes in the genetic
structure of cattle populations across Europe over the past
twenty years.
More recently mini-satellite probes have been used to generate
'genetic fingerprints' which have been used to show differences
between individuals. Such fingerprints have been used to
estimate genetic diversity - the greater the number of bands
revealed by the fingerprint being equated with greater diversity.
This is valid within limits. The main disadvantage of the
fingerprint approach is that the chromosomal location and
number of loci being sampled, and so the proportion of the
genome examined, is unknown. The allelic bands on the gel
cannot be easily identified, so allele inheritance cannot
be addressed making it impossible to trace ancestry.
Through the EC funded BovMaP project, large numbers of highly
polymorphic micro-satellite markers have become available,
which are being mapped on the bovine genome. These markers
are particularly suited to measuring genetic diversity,
and markers can be selected to cover the entire genome.
However, before micro-satellite markers
can be used to examine the structure of the European cattle
population, it will be necessary to standardise the markers
and the protocols for their use to enable results from different
laboratories and countries to be compared.
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