From "G-6-S A genetic Defect and it's managment"-
The affected goats lack an enzyme (G-6-S) and this results in a variety of symptoms of varying severity. The main symptom exhibited by affected goats is failure to grow. Sometimes the kid is smaller than normal at birth, and grows slowly. Some breeders have reported kids which grew normally for the first three months and then stopped growing. Other affected goats grow to what appears to be normal size but is in fact small for the particular bloodlines. They lack muscle mass, appear "slab-sided", sometimes with blocky heads. Immune function appears to be compromised, and sometimes they become deaf or blind. The longest-lived goat known to be G-6-S affected died at just under four years of age, and death is usually due to heart failure. Unfortunately affected animals can and do grow up to breed, although they often experience reproductive problems.
The same symptoms can have many other causes, so that affected animals are seldom recognized as having a genetic defect. Often they grow normally for the first few months and may be sold before any problems become apparent. In that case the breeder may blame the new owner for the goat's failure to thrive and early demise.
Every animal has two genes for every trait, one inherited from the dam and one from the sire. In turn, that animal will pass only one of those genes to each offspring, and which one it will be is a matter of chance, like the flip of a coin. On the average, half the offspring will inherit one gene and half the other. If the two genes are different, then there is a question as to which of them will determine how the animal actually looks or functions. The defective G-6-S mutation is a simple recessive gene, which means that a goat which has only one copy of it will appear perfectly normal and will not show any of the symptoms described above. Such a goat is referred to as a "carrier". A goat which inherits the defective gene from both parents shows symptoms and is referred to as "affected". A "normal" goat, in this context, is one who has two copies of the normal gene.
If a normal goat is bred to a carrier, then all offspring will inherit a normal gene from the normal parent. The carrier parent will pass a normal gene to half the offspring, and a defective gene to the other half. Thus such a mating will, on the average, produce half normal kids and half carriers, and no affected ones. If two carriers are bred to each other, then one quarter of the kids will be normal, one half will be carriers, and one quarter will be affected. If an affected goat is bred to a normal goat, all offspring will be carriers. An affected goat bred to a carrier will produce half carriers and half affected.
As stated above, research shows that 25% of Nubians carry the defective G-6-S gene. Almost all of these are carriers, since most of the affected animals which are born would be culled, and the rest die early. Most people find it surprising that something which is in one quarter of the population can have escaped notice for so long. However, random matings in such a population would result in only one out of sixteen being carrier to carrier, and only one quarter of the kids from these breedings would be affected. Thus only one kid in sixty-four (1.6%) would be affected. Given the variable and obscure symptoms of G-6-S affected kids, it really is understandable that most Nubian breeders believe that they have never encountered affected kids.