Behaviour-genetic analysis of Phormia regina II. Detection of a single, major-gene effect from behavioural variation for Central Excitatory State (CES) using hybrid crosses

For cattle raised in tropical and subtropical environments, production can be limited by the susceptibility of many breeds to parasites. Chemical control, coupled with the use of breeds with higher levels of resistance to parasites, allows beef production to be a viable industry, but at a cost. The Hereford Shorthorn (HS) line of beef cattle at Rockhampton has been used extensively to study genetic aspects of parasite resistance. The hypothesis that a gene with a large effect on resistance to cattle ticks is segregating in this line of cattle has been raised in the past, and is considered again here, with the benefit of data recorded up to the late 1990s. A model in which no major gene effect is fitted provides a satisfactory fit to the data, providing little evidence in support of the segregation of major genes for resistance to either ticks or worms in the HS line. This conclusion is supported by the lack of animals of extreme resistance in later generations, an unlikely result if key animals are assumed to be homozygous in earlier generations.

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Detection of a major gene for litter size in Thoka Cheviot sheep using Bayesian segregation analyses

Animal Science ◽

10.1017/s135772980005311x ◽

2002 ◽

Vol 75 (3) ◽

pp. 339-347 ◽

Cited By ~ 12

Author(s):

G. A. Walling ◽

S. C. Bishop ◽

R. Pong-Wong ◽

G. Gittus ◽

A.J. F. Russel ◽

...

Keyword(s):

Litter Size ◽

Variance Components ◽

Fixed Effects ◽

Major Gene ◽

Single Copy ◽

Threshold Analysis ◽

Data Set ◽

Gene Effect ◽

Inverse Gamma ◽

Major Gene Effect

AbstractSegregation analyses were applied to data from an experimental sheep flock to investigate the presence of a major gene affecting litter size. The data set contained 14 years of litter size data, with up to five parities per ewe, from Cheviot sheep carrying the putative Thoka fecundity gene from Icelandic sheep. Segregation analyses were performed using a Markov chain Monte Carlo method implemented using Gibbs sampling. Uniform priors were initially used for estimating variance components, the gene effect and fixed effects in the data. Genotypes in the base generation were assumed known based on the use of the imported Icelandic donor semen from the founder rams. The use of alternative priors (naïve and inverse-gamma distributions) for the variance components did not significantly affect the results, demonstrating the data to be sufficiently powerful for the analyses used. Segregation analyses detected a major gene for litter size in the Thoka Cheviot flock increasing litter size by 0·70 lambs per ewe lambing for a single copy of the gene. When the analysis was repeated without fixing the genotypes in the base population, the analyses predicted a different genotype than that previously used for one of the founder rams and suggested the major gene to be segregating in the Cheviot founder animals prior to the introduction of the Thoka rams. A liability threshold analysis was also applied to the data. As identified in other studies, the threshold analysis overestimated the heritability, but the estimated major gene effect was not significantly different from other analyses. The results confirm the segregation of the Thoka gene in a Cheviot flock and highlight the statistical method as a useful tool for identifying carrier animals to be used for future matings.

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