Fig 1.
Midline sagittal T1-weighted MRI of brain and craniocervical region of a female GB backcross.
A framework of measured lines and angles is used to assess conformational features associated with CM verified in a previous study [1]. (A) dorsum of spheno-occipital synchondrosis (B) basion of basioccipital bone (C) rostral edge of the dorsal lamina of the atlas (D) junction between the supraoccipital bone and the occipital crest (E) most dorsal point of intersection of the cerebellum with the occipital lobe circle (F) centre of occipital lobe circle placed on the extended cranial baseline (AB) (G) intersection point with the extended AB baseline and DC. The five traits measured in the study are lines ae, bc and f-diameter (blue) and angles FAC (yellow) and AGD (red).
Fig 2.
Study cohort GB family tree (n = 27) including a mixed cross to an Australian terrier and subsequent backcross.
Offspring were designated the combined letters (lowercase) of their parents. Bitch C and Dog F were both a parent and an offspring so designated a single letter as a first generation parent (P1) to ease reading. Hence ed1 is renamed ‘C’ and ‘eh3’ renamed ‘F’.
Table 1.
Summary of CM and SM status in family study group.
Fig 3.
Boxplot distribution of significantly associated traits for CM and SM BVA/KC grades (n = 26).
Top row; 0 = CM0 (normal), 1 = CM1 (intermediate), 2 = CM2 (affected). Bottom row; 0 = SM0 (normal), 1 = SM1 (intermediate), 2 = SM2 (affected).
Table 2.
Descriptive Statistics for five traits in Mixed, Pure and control GB Groups.
Fig 4.
Key morphometric measurements made on the MRIs of dogs in three generation pedigree of the F2 Backcross.
The CM0 F1 hybrid ab1 provides an additional control comparison. The lines and angles have been linked together providing a visual representation of the interrelationships between the individual traits. Sire fg1 and son abfg1 have similar values for angle FAC(~70°) but different values for line bc. Similarly, dam ab4 and daughter abfg2 have similar trait values for line bc (~15.7) but different values for angle FAC.
Table 3.
Morphometric measurements for three generation the F2 backcross pedigree.
Fig 5.
Head conformation and associated angle FAC in six relatives of Bitch H with and without SM.
TW1 sagittal MRI of the caudal fossa and cranial-cervical junctions with superimposed morphometric framework of lines and angles for parents and offspring enhances comparison. The differences in the size of angle FAC are reflected in the lack of skull development caudal to the ear pinna (behind the ears) for dogs with SM compared to dogs with no SM (‘normal’ caudal skull shaded aqua colour). The photos of the heads have been resized to allow comparison using two vertical lines (black) placed at the outer eye and the origin of the external pinna (red) a consistent distance apart.
Fig 6.
Facial features of three generation pedigree of F2 Backcross and F1 and F2 body conformation.
P1 = first parents F1 = first filial generation F2 = second filial generation. P1: Australian terrier (Dam B) has a longer muzzle which is well below eyes (yellow triangle) and upright ear pinna then the brachycephalic GB. Sire A has greater palpebral aperture so the eyes appear proportionally larger to the face. F1 Hybrid: ab4 eyes are “smaller” (more orbital coverage) than the GB with muzzle and ear placement and body conformation intermediate between parents and incorrect for the GB.