【多选题】Monogenic disorders also tend to be very rare, affecting only a fraction of one percent of the human population. This low prevalence can be attributed to a number of factors.
A.
First, many of these diseases are “early onset,” meaning that affected individuals die early in life, often before reproducing. The disease-causing alleles carried by these individuals are thus eliminated from the gene pool.
B.
At the same time, because many monogenic disorders are recessive, heterozygous individuals typically lead normal lives and show no signs of the disease. Thus, the diseasecausing alleles are never entirely eradicated from the population, and instead persist at a low frequency.
C.
The disease then manifests itself only in those rare individuals who inherit two mutant alleles. Monogenic disorders can occur more frequently, however, in families or populations in which the parents are genetically related.
D.
Such consanguineous marriages are more likely to produce offspring that are homozygous for the mutant, disease-causing alleles than are marriages between unrelated individuals.