Genetics 1 & 2
By Karen Hedberg, BVSC - January 1999

Vertebral Column Defects
Selection for any tail, back or head abnormality (from the average normal) is simultaneously a selection for a "midline defect." The dorsal midline of a developing foetus is where the subsequent head, brain, spinal cord and vertebral column arise.

Short/Screw/Stumpy Tails
Selection of a shortening of the tail whether straight or screw tail is selection for a spinal defect as opposed to a straight, normal length tail. This similarly affects Manx cats. Selection for sacro-caudal agensis (short tail/no tail) is associated with an increased incident of spinal dysraphism.

Spinal dysraphism refers collectively to a group of abnormalities involving the spinal cord, vertebral column and the skin subsequent to faulty closure of the neural tube (developing spinal cord). This results in a group of 5-6 different conditions of varying severity of which spina bifida and Hemi vertebrae are the most commonly seen defects.

Spina bifida is a condition where the neural tube (spinal cord) had not fully closed over so that the spinal cord is exposed along a section of the back. In these puppies the vertebral column arch has not closed, neither has the skin. These puppies are euthanised at birth.

Hemi vertebrae are very short/mal-formed vertebrae that can by their abnormality of shape severely distort the spinal cord in affected individuals causing hindquarter paresis/abnormal gait. Severely affected puppies are usually diagnosed by 6 months of age due to increasing pressure on the spinal cord. With the screw tail breeds this can be seen, particularly with the very short backed individuals, with an increased incidence of hem-vertebrae. This defect is usually
associated in combination with brachycephalic (short headed ) breeds.

Brachycephalic (short headed) breeds
The selection for a shorter head, particularly a shorter muzzle, increases the incidence of another midline defect namely cleft palate (and the associated hare lip). Cleft palates can also occur occasionally in any normal headed breed, with the incidence in long headed breeds (Dolciocephalic breeds) being extremely low.

Cleft palate is where the two sides of the roof of the mouth have failed to join during development and is either noticed at birth or within 2-3 days as the puppy cannot suckle (no pressure flatter as the mouth is open to the nasal cavity) and milk comes out of the nose. Failure of the lips to close causes “hare lips”.

Short/screw Tail Genetics
Sailer (1954) - Considered that there were five different inherited tail abnormalities. As many breeds are differentiated by their tail length and carriage, speculation as to the finer points of genetics of inheritance of many of the aspects including length, carriage, etc. are still very open. The inheritance of the stumpy/short tail is considered to be caused by an autosomal dominant gene with incomplete penetrance. Selection for this gene pattern when it occurred in a group of Beagles lead to fewer caudal vertebrae (i.e. shorter tails) and line breeding on these dogs produced cases of spina bifida.

Stumpy/short tails (as opposed to screw tails) were divided by Pulling
(1957) into two groups.

1. Tail-less (anury) or
2. Short stub tail (brachyury)

Selection for excessive shortness of tail would be expected to eventually increase the incidence seen of anal atresia (no anus). This is a separate condition to spinal abnormalities.

Screw tails (Stockard 1941) as seen in English and French Bulldogs was again divided into two possible components.

1. Screw tail gene which is dominant and
2. Length of tail with long screw being dominant to short.

This is still not fully determined but seems likely.

Dermoid Cyst/Sinuses
As a point of interest, while not a vertebral column defect, Dermoid sinuses are a neural tube defect resulting from incomplete separation of the skin and neural tube during development. The sinus of skin tissue runs to attach onto the tip of the vertebrae and in some cases especially posterior locations (pelvic regions) may attach directly to neural tissue. This is considered to be inherited in an autosomal recessive mode.