Is inbreeding a bad thing? Generally speaking the answer is yes.
Then reason is that damaging mutations are more likely to appear in the
homozygous form. This is the reason why it is not lawful to marry someone
to whom you are closely related.
If you carry a damaging recessive allele and your sister does too, then any
children you might have could inherit both and suffer as a result. We all
have defective alleles but if we have a good copy as well as a faulty one we
don't usually suffer the symptoms.
Recent research is challenging this view however. A herd of cattle which has been inbreeding for some 300 years is perfectly healthy. One possibility is that the original population had no defective alleles. This would mean inbreeding did not bring harmful alleles together because there were none. An alternative view is that harmful alleles have been weeded out by natural selection, leaving the functional alleles behind.
Personally I would opt for an original population with very few harmful mutations. This founding population inbred and through natural selection lost the few harmful alleles. Natural selection than maintained the population, weeding out further mutations.
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