INHERITANCE OF RANDOMNESS

The mouse mutant iv is characterized by 'a random determination of a d evelopmental process' in that 50 rather than 100% of the homozygotes h ave situs inversus. The same explanation is given to the inheritance o f situs inversus in the human immotile-cilia syndrome. There are proba bly two alleles of the responsible gene, one for control of the proper asymmetry and one without control and hence resulting in equal number s of situs solitus and situs inversus in the homozygote. Left-handedne ss may be similarly inherited; furthermore, because of its high preval ence (around 10-12%) it has been assumed that there is an advantage of carrying both alleles ('balanced polymorphism'). With these two assum ptions, a prevalence of left-handedness of 12.5% is expected, a 50% ch ance of left-handedness in matings between two left-handed persons, an d equal numbers of discordant and left-handed concordant monozygotic t wins. These values are close to those actually found. As the values fo r (female) homosexuality are similar to those of left-handedness a sim ilar inheritance is proposed.