This is from Clare Rusbridge's blog and has an interesting diagram which illustrates why EBVs will give even those breeders that " know what is behind their dogs" better information on which to base their breeding decisions

Effectiveness of breeding guidelines for reducing the prevalence of syringomyelia

S. P. Knowler, A. K. McFadyen, C. Rusbridge
Veterinary Record 2011;vetrec-2011-100062

Several toy breed dogs are predisposed to syringomyelia (SM), a spinal cord disorder, characterised by fluid-filled cavitation. SM is a complex trait with a moderately high heritability. Selective breeding against SM is confounded by its complex inheritance, its late onset nature and high prevalence in some breeds. This study investigated the early outcome of existing SM breeding guidelines. Six hundred and forty-three dogs, 550 Cavalier King Charles spaniels (CKCS) and 93 Griffon Bruxellois (GB), were identified as having either one (454 dogs) or both parents (189 dogs) with MRI-determined SM status. Offspring without SM were more common when the parents were both clear of SM (SM-free; CKCS 70 per cent, GB 73 per cent). Conversely, offspring with SM were more likely when both parents had SM (SM-affected; CKCS 92 per cent, GB 100 per cent). A mating of one SM-free parent with an SM-affected parent was risky for SM affectedness with 77 per cent of CKCS and 46 per cent of GB offspring being SM-affected. It is recommended that all breeding dogs from breeds susceptible to SM be MRI screened; that the SM status at five years old is established; and all results submitted to a central database that can be used by dog breeders to better enable mate selection based on estimated breeding values.
For example

This family tree typifies the complex relationship between SM-affected asymptomatic dogs and SM-free dogs and demonstrates how hard it is for breeders to make sensible breeding decisions even when they are knowledgeable about their dogs’ background.
The figure illustrates a three generation CKCS family tree based on the SM grades used in the 2006 International Breeding guidelines ( - see under SM screening) *A denotes a dog that has been screened over the age of 5 years and is clear of SM. Square = male, circle= female.

The family group is centred on a grade A male (red) without a central canal dilatation (CCD) who was 3.8 years when he had the MRI. He has an *A grade dam and a sire of unknown status. He was mated to two unrelated grade D females. Three of the offspring from one parental cross had early onset SM (E grade) however the two offspring from the other parental cross were both A grade. It is perhaps significant that for the E grade litter both maternal grandparents were D whereas the maternal grandmother of the A grade litter was *A. Not all breeders may be afford to MRI more than a few dogs and may not be provided with the MRI status any relatives of a potential mate. The EBVs for SM can be combined with those for MVD and other genetic diseases thereby providing more accurate risk information at birth and eliminating environmental influences they provide the best way to help the cavalier breed.