Monozygosity in twins: a detailed investigation.

The twins presented in this article showed similarities in their physical make-up to a very high degree. Statistically, however, it is the blood picture which holds the "trump card" in this plethora of evidence supporting monozygous twinning, and it is an essential prerequisite in any presentation of this nature. It is perhaps noteworthy that, while only one of the twins sucked his thumb habitually, there was no difference in the degree of postnormality of the posterior teeth but merely a change in the interincisor relation and in the palatal height. It can clearly be seen from the study of these two twins that the differences between them were very small indeed and could quite easily be classed as individual variations within the same identical genetic pattern. Similarly, in any other identical twins, the way in which one differs from another must be to a similarly small degree or the cause of the difference must clearly be attributable to environmental or acquired influences. Differences which cannot be classed as other than genetic show us that the twins are fraternal or dizygous and, therefore, no more alike than would be two brothers of differing ages.