It is
common knowledge that
inbreeding results in
genetic problems; inbred
offspring are likely to suffer from multiple
genetic defects and are often subject to
mental retardation or
disease.
All organisms have some of the genetic flaws that cause these syndromes. These alleles rarely manifest themselves in the phenotype, however, because they are recessive; there is usually a dominant allele to counter the negative effects. But when these genes mix with close relatives, the frequencies of the rare, harmful recessive alleles increase; they eventually become common within the inbred population. Even if a particular bad allele fails to dominate, the gene pool is easily big enough to supply replacements. The genetic defects are compounded in this way until the offspring have serious problems; often the inbred offspring do not survive. A famous example is the hemophilia that arose among the royal family of England. After generations of inbreeding, hemophilia suddenly became very common within the family; many of Queen Victoria's children suffered from this disorder, most notably Prince Leopold, who died young from a short fall.