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Lack of diversity in the gene pool.
say you have a "bad" allele for gene X. if that allele is recessive, and you also have the dominant "good allele," then you will be OK. HOWEVER, if you have offspring with somebody who is related to you (meaning, of course, that they have similar genes as you), there's a possibility that they will have that same "bad" allele, and transfer it to your offspring. At the same time, YOU could transfer YOUR bad allele to your offspring, and since your offspring would have two recessive "bad" alleles, it would receive the "bad phenotype." You can reduce this chance by having offspring with somebody who is more genetically DIFFERENT than you, which is why, for example, we are not attracted sexually to our family members.