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<blockquote data-quote="ANAZAZI" data-source="post: 688713" data-attributes="member: 7541"><p>Really, the heterosis is the kick upwards relative the expectation of what the combination of dam and sire would perform based on their performance.</p><p></p><p>Now, the natural selection and the breeding that people do to cattle; they both have a tendency to "punish" dominant "bad genes" much harder than recessive "bad genes" because of the recessive genes ability to hide behind "good" genes. Because of this tendency, most alleles that limits health, growth and performance happens to be recessive, as the dominant alleles that cause the same kind of problem are already extinct.</p><p>This tendency also makes it more likely that within a restricted population such as a breed, there are recessive alleles of the "bad kind" that sometimes can "come out and play", because they in a particular animal are homozygous. now figure that there are ten or a hundred such alleles within a breed, this pulls down the breed average by roughly a few percent.</p><p>And in the other breed the same thing happens, however with other "bad genes" that happens to be present, but with quite the same results.</p><p>If these two breed are crossed; it is likely that many of these "bad genes" are masked by "good genes" from the other breed.</p><p>There is only a slight heterosis effect when crossing two lines within a breed because they are likely to share most of their "crap".</p><p>When you mix hereford and aberdeen angus for example, their gene pools has been apart for a few hundred years, they are less related and tend to share less of the "crap".</p><p>Now, if you breed your herf to brahma bull there will be more heterosis, because the more difference in the gene pool the lees "bad genes" they will share.</p><p>A composite breed will have a much larger gene pool than a traditional breed, and therefore much lesser risk that one of the recessive genes "finds" a copy of "itself" and is expressed in the phenotype.</p></blockquote><p></p>
[QUOTE="ANAZAZI, post: 688713, member: 7541"] Really, the heterosis is the kick upwards relative the expectation of what the combination of dam and sire would perform based on their performance. Now, the natural selection and the breeding that people do to cattle; they both have a tendency to "punish" dominant "bad genes" much harder than recessive "bad genes" because of the recessive genes ability to hide behind "good" genes. Because of this tendency, most alleles that limits health, growth and performance happens to be recessive, as the dominant alleles that cause the same kind of problem are already extinct. This tendency also makes it more likely that within a restricted population such as a breed, there are recessive alleles of the "bad kind" that sometimes can "come out and play", because they in a particular animal are homozygous. now figure that there are ten or a hundred such alleles within a breed, this pulls down the breed average by roughly a few percent. And in the other breed the same thing happens, however with other "bad genes" that happens to be present, but with quite the same results. If these two breed are crossed; it is likely that many of these "bad genes" are masked by "good genes" from the other breed. There is only a slight heterosis effect when crossing two lines within a breed because they are likely to share most of their "crap". When you mix hereford and aberdeen angus for example, their gene pools has been apart for a few hundred years, they are less related and tend to share less of the "crap". Now, if you breed your herf to brahma bull there will be more heterosis, because the more difference in the gene pool the lees "bad genes" they will share. A composite breed will have a much larger gene pool than a traditional breed, and therefore much lesser risk that one of the recessive genes "finds" a copy of "itself" and is expressed in the phenotype. [/QUOTE]
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