7.3 Measures of Genetic Variation

Coefficients of inbreeding and relationship [WrightWright1922] have been commonly used to describe the genetic diversity in livestock populations [Young and SeykoraYoung and Seykora1996]. Inbreeding coefficients represent an individual's expected genetic homozygosity due to the relatedness of its parents. Coefficients of relationship describe the expected proportion of genes two individuals share due to their relatedness. These are relative measures that depend on such factors as the completeness and depth of pedigrees. Over time, these coefficients change in response to breeding and culling decisions, and they may be used as indicators of the genetic variability of a population. Rapid methods for calculating coefficients of inbreeding and relationship for large populations have been implemented [Wiggans, Van Raden, and ZuurbierWiggans et al.1995].

Populations under study rarely conform to the theory established for the use of coefficients of inbreeding [WrightWright1931]. Lacy ref640 and Boichard et al. ref352 proposed measures of genetic variation based on ideas from conservation genetics. Lacy ref640 proposed the idea of the number of founder equivalents in assessing populations. A founder is an ancestor whose parents are unknown. If all founders contribute to the population equally, then the founder equivalent is equal to the number of founders. When founders contribute unequally to the population, the number of founder equivalents decreases. Boichard et al. ref352 developed the idea of founder ancestor equivalents, which is the minimum number of ancestors necessary to explain the genetic diversity of the current population. Founder ancestor equivalents account for bottlenecks, unlike founder equivalents, and are more accurate in populations undergoing intense selection. Caballero and Toro ref817 discussed the relationships among these and other measures of diversity in small populations, and demonstrate their use [Toro, Rodriganez, Silio., and RodriguezToro et al.2000].

Roughsedge et al. ref641 used average coefficients of inbreeding, average coefficients of relationship, founder equivalent numbers, and founder ancestor numbers to document the decrease in genetic diversity in the British dairy cattle population over the last 25 years. Changes in founder equivalent number and founder ancestor number reflected the use of a small number of influential individuals to improve the genetic merit of that population. Accompanying changes in average inbreeding and relationship did not accurately reflect that loss of diversity. Such results highlight the need for additional tools when assessing complex populations.

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