Effects of inbreeding on four families of Peppin Merinos. IV. The expression of heterosis

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Abstract
Whereas Inbreeding in Merino sheep seems to have an adverse effect on many fitness characters, it does not appear to limit the potential maternal ability of the ewes in respect to the rearing of a single lamb. There is, however, a complex effect on fertility such that inbred ewes appear to be more variable in their response to environmental fluctuations in terms of successful pregnancies. The variance of body weight is significantly increased by inbreeding, as would be expected following a disturbance of the "developmental homeostasis". However, the variance of fleece weight, which is a character strongly depressed by inbreeding, does not appear to be affected. It is suggested that the 'inbreeding syndrome' is related to excess homozygosity but that the variance changes in the depressed characters result from the consequent disturbance of physiological mechanisms. Outcrossing, both between two Inbred lines from the same strain and between inbred lines and unrelated sheep of the same strain, produces the same results, namely, a reversion of the progeny to the normal characteristics of the original random-bred populations.

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