The reason that incest today often results in genetic abnormalities is that when two people of similar genetics (i.e., a brother and sister) have children together, there is a high risk of their recessive characteristics becoming dominant.
Sababu ya madhara ya ndoa za kaka na dada ni kwamba kama kila mmoja wao alikuwa na hali Fulani ya kilema ndani yaoya kurithi basi atakayezaliwa atakuwa mwenye kurithi kutoka kwa wote wazazi wawili, hivyo basi kuwa hali mbaya zaidi kuliko waliomzaa.