There are certain facts with regard to mongolism which must be explained by any hypothesis intended to account for this condition. No hypothesis heretofore presented will, in an unmodified form, avoid some conflict with available knowledge. These facts may be briefly summarized as follows:
When mongolism occurs in a pair of twins of the opposite sex, only one twin is affected.
When mongolism occurs in a pair of twins of the same sex, which are evidently dizygotic, only one member is affected.
When mongolism occurs in a pair of monozygotic twins, both members are affected.
Mongolism rarely occurs in more than one member of the same family.
Mongolism has been known to occur in three or even four members of a sibship.
The incidence of mongolism varies widely as a function of the age of the mother. Mongolian idiots1 are progressively more prone
JENKINS RL. ETIOLOGY OF MONGOLISM. Am J Dis Child. 1933;45(3):506–519. doi:10.1001/archpedi.1933.01950160048006