9.2 Effect of crossbreeding
The effect of crossbreeding is the opposite of the effect of inbreeding. Crosses are more heterozygotic than individuals in a pure bred population.Contrary to inbreeding, where a linear negative relationship with fitness and production traits and the degree ofinbreeding, it is impossible to predict anything about the effect ofcrossbreeding.
How is it possible to establish a sensible system of crossbreeding? The only possible method is 'the trial and error method'.Today a large part of the practical production of poultry,mink and pigs is based on production crosses, so the 'the trial and error method' is notas bad as it might sound. If a combination,showing a good crossbreeding effect or heterosis, is found, this combination can be repeatedinfinitely. Apparently, a certain heterosis does not depend on the level of purebreeding. Pure breeding or out breeding has not been defined earlier, but the words indicate that neither inbreeding nor crossbreedingoccur, therefore breeding in alarger population of domestic animals is referred to as pure breeding or outbreeding. This definition corresponds closely to the definition of a H-Wpopulation with less strict random mating requirements. Hybrid vigour is, asit says, a special vigour, which occurs as a consequence of crossbreeding. Another wordused for a cross is a hybrid, which has then coined the term. Vigour is materialized in more robust animals withlarger productioncapacity than pure breeds. Heterosis means the same as hybrid vigour. The word indicates that the effects arecaused by increased heterozygosity.
Figure 9.2. Systems for crossbreeding. 
Systems for crossbreeding. Figure 9.2 shows four breeds or lines, A, B, C and D.Crossbreeding can be carried out as two way, three way or four way crosses, backcrosses, or rotationcrosses.
Two way crosses - a cross between two lines A and B.
Back crosses - the crossed animal, AB, mated with either line A or B.
Three way crosses - the crossed animal, CD, is mated with a third line A.
Four way crosses - the crossed animal, AB, is mated with the crosses CD.
Rotation crossbreeding can be carried out with 3, 4 or 5 breeds or lines. Inthis case 4 is used, the next breed to be used is C and then D. Normallythe females are crosses while the males are pure bred animals.
The two-way crossbreeding only gives heterosis in the offspring. To retain heterosisin the maternal traits, three way or four-way crossing has to be applied.
Systems for crossbreeding automatically ensures as low an inbreeding as possible in the productionanimals. Thus the advantage of crossing systems is that no specialaction has to be taken in order to maintain minimal inbreeding.
The traits, which give largest hybrid vigour, are traits with low heritability, which implies that hybrid vigour is mostlyfound in reproduction traits.
Figure 9.3 shows an example of hybrid vigour in mice on the number of younga female can produce in a lifetime. The most popular system for crossbreedinghas been applied: two- and three- way crosses, and back crosses. (Data from Newman et al. J.Anim. Sci. 61 358-365, 1985).
Figure 9.3. Effect of crossbreeding in mice.
As seen in Figure 9.3, the hybrid vigour is very high in this example as miceof pure lines only get half the number of young that the cross bred mice get.
The effects can be divided into either an effect on the foetus or an effect on the mothering abilities.The difference between the two lower curves shows the effect on the foetus,if it is a cross or not. The lower curve represents pure bred foetusesin pure bred mothers. The second lower curve represents a cross foetusin a pure breed mother. The difference between the two lower curves and the twoupper ones shows the effect of a female being a cross or not. It is clear that most of the hybrid vigouris in the maternal part, and it concerns the ability to ovulate a large numberof oocytes and hold them throughout the pregnancy.
The example in Figure 9.3 shows a large amount of hybrid vigour. Thisis very rare, but in animal breeding it is common to see effects ofup to 10 %.
Calculation of the hybrid vigour is done as follows: (average of the crosses minus average of the pure lines) in relation to the average of the pure lines.
Example of swine: Landrace Yorkshire crosses give an extra piglet per litter compared to the pure bred sows, whichget an average litter size of 10.
Hybrid vigour (11-10)/10 = 0.10 or 10% for litter size in swine in the mentioned combination.
To carry out a cross production program it is necessary to maintain well defined lines, and thatthe animals are bred in sufficiently large numbers both tomaintain them self and to produce a sufficient amount of animals to get into thecrossbreeding program. The necessary surplus of animals for breeding is not always present. Thishowever isnot the case in the Danish dairy breeds. Largely all females are used forbreeding, as the birth rate for the Red Danish and for the Holstein Frisian isonly slightly above two. A ratio of 1.1 to 1.2 heifer calves have to born inorder to replace their mothers, so the birth rate is just high enough to maintain the population. Rotationcrossing though can be practised outside the pure breeding kernel. The hybrid vigour is significantlylower in rotation- than in three way crossbreeding.
An extra advantage using crossing is the uniformity, whichis attained when at leas one of the parents is a pure bred animal. In a German investigation was found that the phenotypic variance inback fat thickness in swine was 30 per cent lower in L x Y crosses than in the corresponding pure bred animals (Lutaaya et al. 2001, 79:3002-07).
Figure 9.4. Two gene pairs and possiblebiochemical pathways. Newcombinations are not allwaysbeneficial.
Negative recombination effect in F2: In the showncrossing systems, at least one of the parent breeds was a pure bred. If an F2 is produced,this is not thecase. Production based on F2-animals or crossbreeding between different crosses is normally a bad ideacompared to backcrosses or three way crossing. The latter has the largest heterosis effect. In F2sso-called negative recombination effects can occur. This iscaused by combinations of genes, which did not exist in any of the original breeds.For instance if a F1 is made by crossing aabb x AABB in F2animalsof the type aaBB AAbb will occur, which were not found in any of the originalbreeds. These types can be lethal, see Figure 9.4 for the biochemical pathways.The F2 will furthermore be more heterogenous than all other crosstypes or purebreed animals.
By incrossing it is possible to improve a breed by using a few superior animals from outside the breed. Normally animals from closely related breeds areused because they have the desired characters to save the best of the originalbreed. For this to succeed the selection must be light in the first generations of the incrossing. After incrossingsome of the recombinant types are of higher interest and it can take several generationsfor them to occur
Figure 9.5. A pair of hom*ologouschromosomes from an original and anincrossed individual before recombination.
Figure 9.5 shows hom*ologous chromosomes with four gene pairs which affect a quantitative character. Genes can be mixedso that it takes several generations for the desired combination, one chromosomehaving all the plus signs, to occur. See also section 2.5 for linkage disequilibrium.
