Inbreeding may result in a far higher phenotypic expression of deleterious recessive genes within a population than would normally be expected.^[1] As a result, first-generation inbred individuals are more likely to show physical and health defects, including:

• Reduced fertility both in litter size and sperm viability
• Increased genetic disorders
• Fluctuating facial asymmetry
• Lower birth rate
• Higher infant mortality
• Slower growth rate
• Smaller adult size
• Loss of immune system function

Natural selection works to remove individuals who acquire the above types of traits from the gene pool. Therefore, many more individuals in the first generation of inbreeding will never live to reproduce. Over time, with isolation such as a population bottleneck caused by purposeful (assortative) breeding or natural environmental stresses, the deleterious inherited traits are culled.

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The most famous example of a genetic disorder aggravated by royal family intermarriage was the House of Habsburg, which inmarried particularly often. Famous in this case is the Habsburger (Unter) Lippe (Habsburg jaw/Habsburg lip/"Austrian lip"), typical for many Habsburg relatives over a period of six centuries.^[19] The condition progressed through the generations to the point that the last of the Spanish Habsburgs, Charles II of Spain, could not properly chew his food.^[20] (See mandibular prognathism.)

Besides the jaw deformity, Charles II also had a huge number of other genetic physical, intellectual, sexual, and emotional problems. It is speculated that the simultaneous occurrence in Charles II of two different genetic disorders: combined pituitary hormone deficiency and distal renal tubular acidosis could explain most of the complex clinical profile of this king, including his impotence/infertility which in last instance led to the extinction of the dynasty.^[21]

The most famous genetic disease that circulated among European royalty was hemophilia. Because the progenitor, Queen Victoria, was in a first cousin marriage, it is often mistakenly believed that the cause was consanguinity. However, this disease is not aggravated by cousin marriages.