The disease is called GHN and is a newly discovered genetic defect. It affects puppies from 3-9 months and below a short description of the clinical signs and the genetic inheretence.

The text is borrow from Dr Barbara Kessler. 

Clinical Symptoms

Usually, first signs of the disease occur at an age of 3-4 months, in

some cases not before 8-9 months. Affected puppies quite suddenly show

an abnormal gait with arched back and rear weakness, which may suppose

an injury in loin. Within a few days or weeks, condition gets worse,

the puppies visibly loose muscle and hardly can walk or run for more

than 200-300 metres. Even just a few steps lead to fatigue, puppies are

no more able to fully stretch the knee joints. They show a strange,

"bunny-like" hopping gallop with turned-out knees, later on they

collapse and cannot walk anymore. After a few minutes of recreation,

they recover strength and can continue walking for another short

distance.

  In addition to the abnormal gait, neurologic deficits come to the fore

of the pathogenesis. The spinal reflexes are highly decelerated resp.

totally absent, especially the patellar tendon reflex. In contrast,

sensoric and proprioreceptive functions are usually not affected.

With increasing age, muscle weakness is more and more progressive, the

gait gets wobbling and unstable, up to the dogs cannot get up and walk

without help any more. It differs from case to case which muscle groups

are particularly affected. Usually shoulder and thigh, as well as

temporal, back and in some animals also the laryngeal muscles are

highly atrophic. The latter are standing out because they cannot bark

and are sometimes dyspnoeic.  Affected puppies don't show any sign of

pain or disturbed well-being, they are alert and full of joie de vivre

until the end. However, they only reach an age of 9 to 13 months until

they have to be put to sleep because of their progressive muscle

atrophy.