SOLOMON
92
AND
KASSAHUN
different breed combinations. The occurrence of heterosis is directly proportional to the degree of hetrozygosity.
Examples Example 1
Example 2
Performance of a crossbred : additive gene action
Performance of a crossbred: the effect of heterosis
• Post-weaning growth: • Breed A = 100 g/day • Breed B = 140 g/day.
• Expected post-weaning growth: • Crossbred = 120 g/day.
• Post-weaning growth: • Breed A = 100 g/day • Breed B = 140 g/day • Crossbred = 132 g/day
• Average of breeds: • A + B = 120 g/day
• Difference (estimate of heterosis): • = (132–120) • = 12 g/day
Heterosis can be to the extent that the crossbreds could show performance over both of the parents. This is important if farmers plan to mate two breeds where each is successful in the locality (two different indigenous breeds) with the desired result of the crossbred being superior to the two foundation breeds. If this is not the case, it would be more sensible for farmers to replace the poorer of the two breeds by the better. This can be done directly or by grading-up. The expression of heterosis is always at its maximum (100%) in the first cross between two breeds (F1). Varying amounts of the heterosis are lost in later generations of crossing because some of the hetrozygosity in gene pairs is lost, which is called recombination loss.
Reciprocal crosses For accurate comparison of crossbred performance with purebred performance, it is theoretically required that the cross should have been made in both of the two possible ways:
• females of breed A (e.g., local breed) mated to males of breed B (e.g., an exotic breed); or • females of breed B mated to males of breed A. These two variants are called reciprocal crosses. Though genetically alike, they differ because the reciprocal crosses have had a different maternal environment: one from dams of the local breed (breed A), the other from dams of the exotic breed (breed B). These maternal influences can be important for the offspring at the time of birth and, perhaps, up to the time of weaning. After weaning, the importance of the maternal effect usually lessens but sometimes never disappears completely. The effect on the offspring arises because different maternal environments may provide the fetus and, later, the newborn animal with different advantages at the start of life. One breed may supply the crossbred offspring with better nutrition even before birth and may have better mothering abilities thereafter. Locally adapted dams may give the newborn a better supply of antibodies in the colostrum than dams of a recently imported or exotic breed.
Ethiopia Sheep and Goat Productivity Improvement Program