Defense Date


Document Type


Degree Name

Master of Science



First Advisor

J. Ives Townsend


The method of principal components may be used to reduce a large quantity of data for an individual into a single statistic, a growth index, indicative of overall facial growth, and to make a determination of the relative contribution of each variable, as well as the genotype and the environment, to the variation between individuals, When this method is applied to four consecutive years of cephalometric and anthropometric data from each of 95 children, consisting of Caucasian and Negro monozygotic twins, like-sexed dizygotic twins, and their siblings of both sexes, it discloses that:

1. The relative contributions of the variables studied to the variation among individuals are as follows: facial depth variables > facial height variables > facial width variables.

2. No differences in growth rates, as represented by the growth index, are apparent between males and females entering the study during the same age interval, between members of the same sex entering the study during two age intervals, between sexes including all age intervals, and between races. Failure of the investigation to disclose any such differences may result from the design of the experiments.

3. A very highly significant environmental component of variability for the growth index is found in the population studied, which suggests the need for further studies of specific environmental agents affecting the growth rates of the variables involved.

4. A very highly significant genetic component of variability for the growth index also is found, however, the complicated polygenic nature of facial inheritance renders analysis of the specific genetic factors involved quite difficult, because of the present limited knowledge of the inheritance of quantitative traits.

5. A very highly significant extrafamilial (genetic and environmental) component of variability for the growth index also is found.

6. The sources of variability for the growth index for twins, have the following relative magnitudes: extra-familial > genetic > environmental > error. For siblings the relative magnitudes of the sources of variability are: extrafamilial > environmental> genetic > error. Thus, as expected, environmental factors are relatively more important between siblings than between twins.


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