THE USE OF ANTHROPOLOGICAL TRAITS AND BLOOD GROUPS IN THE DETERMINATION OF THE ZYGOSITY OF TWINS

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Abstract
The present work is based on unselected twin material originally collected and studied by Essen-Moller 20 years ago. From this Essen-Moller sorted out a group of proven monozygous pairs, diagnosed by the presence of monochoria, and a group of proven dizygous pairs of the same sex, distinguished by means of blood group differences. By studying the intra-pair similarities and dissimilarities for 15 anthropological traits, Essen-Moller was able to provide proof of the tenability of the polysymptomatic similarity test, and to express the diagnostic value of the 15 traits numerically. By use of 8 more blood group systems the present investigators increased the group of proven dizygous pairs considerably, thus extending the basis for determining the value of the anthropological characters in the zygosity diagnosis. The iris, the color of the hair and the finger prints give very valuable information, although an intra-pair difference does not give definite proof of dizygosity as does a difference in blood groups. The use of many of the anthropological traits is limited because side by side comparison of the partners is necessary. Further, the numerical expressions of their diagnostic value given here do not apply to populations with a different distribution of the traits. The blood groups, on the other hand, may provide a proof of dizygosity in more than 95 per cent of the same-sexed dizygous pairs if most or all of the systems are used. When the frequencies of the corresponding genes are known, as is the case in most countries, the probability of monozygosity in same-sexed pairs of identical blood groups may be calculated without any special preceding twin investigations being necessary. Blood groups may also be used in twin pairs of which the partners cannot be examined at the same time or place. A comparison between various diagnostic methods was undertaken in order to give an estimate of the number and direction of misclassifications.