Utilisation and conservation of farm animal genetic resources 73
Chapter 3. What is genetic diversity?
interpretation of this measure in terms of evolutionary forces is diicult in livestock because selection plays a major role in breed development and neutrality of easily
measured quantitative traits conformation, production, reproduction is perhaps more questionable than anonymous DNA markers. Q
ST
can be expressed as σ
B 2
σ
B 2
+2σ
W 2
, so high values of Q
ST
indicate a high degree of diferentiation between breeds, and consequently the importance of breed variation. It is closely related to
the direct measure g
1
=σ
B 2
σ
B 2
+σ
W 2
based upon extant genetic variation since Q
ST
= g
1
2-g
1
. For example, using the values of g
1
in Box 3.2, estimates of Q
ST
for feed conversion and relative growth rate were 0.14 and 0.20.
4. Probabilities of ixation and loss of individual alleles from a population and hence the number of alleles we observe will depend on migration, drit and also selection if
the allele is not neutral. he extent of drit, and consequently how these probabilities will change over time, will depend on ΔF. his is covered in more detail by Crow and
Kimura 1972, and results have also been developed for selection e.g. Caballero et al., 1996.
8. Conclusion
In ‘he Name of the Rose’ by Umberto Eco 1992, as translated by William Weaver, Father William of Baskerville delights in the diversity of nature by declaring ‘the beauty
of the cosmos derives not only from unity in variety, but also from variety in unity’. He summarises in just 18 words − and only 13 distinct words − that when examining
the population of a species we should expect variety, and will invariably ind it, and as we look closer at what appears at irst sight to be a more uniform sub-population
of individuals, so variety remains. However, quantifying diversity, understanding its scientiic nature and importance, and providing guidance on how to use and conserve
it, requires many more words
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