advantages and disadvantages of line breeding and out crossing
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Advantage and disadvantage of cross breeding
The major advantage of crossbred dairy cattle is that they exhibit the strengths of all breeds from which they descend with an added advantage of heterosis. Heterosis tends to be most important for lowly heritable traits such as fertility and survival. The performance for a particular crossbred for a trait will be a combination of the breed merit for that trait of the breeds which make up the crossbred and the heterosis for that trait which is expressed in the crossbred. That combination can be higher than the breed merit for that trait of the superior breed in the crossbred's makeup.
The major disadvantages are that crossbreds also have the weaknesses of the breeds from which they descend and heterosis in initial crosses declines with any backcrossing to parental breeds. Rotational crossbreeding plans, particularly with three breeds, can maintain substantial heterosis, but maintaining a rotational crossbreeding program requires careful record keeping and planning.
One should be aware when crossbreeding that with early generations of crosses, there may be considerable diversity with regard to size, body condition, and other traits, depending on the breeds utilized.
Benefits of crossbreeding depend on good sire selection within the pure breeds, just as purebreds depend on within-breed selection for genetic improvement.
Line breeding
Line breeding attempts to gain the benefits of inbreeding while reducing the risks.
It does so by accumulating multiple crosses of one or more superior ancestors, typically at the fifth generation and beyond.
The hope is that through selective breeding and weeding out of undesirable traits over several generations, the foal can inherit a wider selection of the ancestor's beneficial genes through several of the ancestor's good sons and daughters.
Line breeding to multiple good ancestors in a pedigree tends to indicate compatibility between the sire's and dam's ancestry.
The great stallion Northern Dancer is an excellent example of a line bred pedigree. While on the surface he was only mildly inbred with a 4x5 cross to Gainsborough (one cross at the fourth generation and one at the fifth), he had 23 crosses to the great English racer and sire St. Simon within the sixth through eighth generations of his pedigree.
He also had five crosses (6x6x6x8x7) of the important mare Canterbury Pilgrim; a 4x6 cross of another important mare, Selene; a 4x6 cross to the top sire Polymelus; and a 5x6 cross to Derby Stakes winner Spearmin
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advantage and disadvantage of line breeding in plants