Consanguineous matings are more likely to produce offspring homozygous for a recessive trait. This increase in probability results from the fact that individuals with recent common ancestors have a greater possibility of inheriting the same recessive alleles than do unrelated persons. The extent to which human consanguinity increases the incidence of inherited diseases is still debated by geneticists. Some feel that zygotes homozygous for deleterious mutations are affected so severely that most are spontaneously aborted before birth. Some geneticists also feel that consanguinity is just as likely to concentrate favorable alleles as deleterious ones.
Most cultures have laws or taboos which prevent marriage between closely related adults.
This may be the result of observations that stillbirths and birth defects are more common when parents are closely related. Consanguinity may not always be harmful. Additionally, carefully planned breeding experiments in some zoo populations is preventing the extinction of some rare species. Some human disorders are inherited via dominant alleles. In medicine, an allele is classified as dominant if a single copy is sufficient to significantly affect the phenotype.