Statistical Models for Landmark Coordinate Data - An Invariant Approach to Statistical Analysis of Shapes - page 79

instability), but the distinct craniofacial appearance seems to occur in
all children with DS. Though present in all individuals with DS, the
characteristic facial features are highly variable. Since these features
most likely arise due to differences in the developmental regime of
euploid (non-trisomic) and trisomic individuals, an animal model pro-
vides certain advantages to the study of the developmental basis of
these facial features since it can be manipulated at any time during
development.
The Ts65Dn mouse is a model for DS. To establish the value of the
Ts65Dn mouse model in the study of DS, initial energies were directed
towards definition of the various phenotypes of interest that are
expressed in the animal model. We sought to define the craniofacial
phenotype. Using the Reflex microscope, three-dimensional landmark
coordinate data were collected from the mandibles of seven adult
Ts65Dn mice and 13 adult euploid littermates. Landmarks identify loci
on the exterior surface of the left hemi-mandible (see Figure 3.2 ) . These
data were used to calculate a mean form, a mean form matrix, and a
variance-covariance matrix for both the euploid and aneuploid samples
using the methods described above and presented in detail in Part 2 of
this chapter. The estimated parameters are given in Table 3.1a- f .

Table 3.1a. Mean form of the aneuploid Ts65Dn mandible ( mm ) (Coordinate
values correspond to the mean form matrix)
Landmark (number) X Y Z

coronoid process (1) 2.312 2.582 0.408

mandibular angle (2) 5.185 -2.875 -0.044

anterior aspect of condyle (3) 4.702 1.517 -0.070

posterior aspect of condyle (4) 6.231 0.631 -0.460

high point on mandibular body (5) 0.220 -1.872 0.308

posterior aspect at base of alveolar thickening (6) -1.902 -2.221 -0.113

posterior aspect of incisor at alveolus (7) -5.192 -0.795 -0.126

anterior aspect of incisor at alveolus (8) -5.552 1.438 -0.496

body of alveolar ridge (9) -2.971 -0.269 0.325

anterior aspect of M1 on alveolus (10) -2.361 0.965 0.039

intersection of coronoid -0.672 0.901 0.229

process and alveolar rim (11)