Greek temples , tropical kine and recombination load

New breeds typically arise from the cross of two or more existing breeds, often chosen to complement each other. For about 100 years, efforts have been made in the tropics to produce a milking breed combining zebu tropical adaptation with taurus production ability. All attempts have failed, although the F in nearly all instances has been superior. The Greek 1 Temple Model (GTM) is a graphical representation of crossbreeding theory which completely accounts for intralocus genetic effects, but not epistatic effects. All crosses except the F may lose merit due to destruction by recombination at meiosis of 1 favorable epistatic combinations built up during reproductive isolation. Data from a meta analysis of 80 reports of taurus-indigenous breed crosses in the tropics were used to fit the GTM and to estimate recombination load. Data for lactation length and calving interval fit the GTM (P ̄ 0.3) indicating that epistasis was not an important source of genetic variance in the crosses; however, data for lactational milk yield, age at first calf and annual milk yield did not. The GTM was extended to include recombination load under a two-locus model, and satisfactory agreement was found for the remaining 2 traits (0.6,P,0.9). Minimum x estimates of recombination load for lactational milk yield and annual milk yield were 2277655 kg and 2345656 kg, respectively. Three kinds of loads were necessary to account for breed cross means for age at first calving. Heifers having an intact taurus but impaired zebu complex were predicted to calve about 3.561.5 months earlier while those having the alternative combination were predicted to calve 2.868 months later. Heifers lacking either ancestral complex calved 8.967.2 months later. Plausibility of the results was discussed in light of recent evidence suggesting at least a quarter-million-year separation of the two races of cattle. Long periods of time would be necessary for interaction systems to have evolved differently and produce load upon crossing. It is concluded that the 100-year effort to breed a tropically adapted dairy cow from a zebu3taurus base failed, not because of lack of effort or tenacity or faulty methodology, but because recombination between incompatible genetic interaction systems caused too great a load on the crossbred generations. In vitro fertilization and allied technologies are suggested as a way to exploit the obvious advantage of the F and to circumvent genetic load.  2001 Elsevier Science B.V. All rights reserved. 1