"Normal" Development with a Genetic Disorder

[D Martin]

How does a person with a genetically-based degenerative disorder develop apparently normally from conception, through birth and often into advanced adulthood? (Muscular dystrophy often manifests in early childhood, arrhythmogenic right ventricular cardiomyopathy in the teens, and Parkinson's after 50.) One would think the genetic variation/mutation would not allow for normal development. 

[iNow]

8 hours ago, D Martin said:

One would think the genetic variation/mutation would not allow for normal development. 

Unless it prevents you from successfully mating with a partner and producing offspring that survive and also reproduce, there's nothing stopping it from carrying forward in the population. 

[Genady]

I understand that the OP question is about ontogeny of organism with a genetic disorder. What I don't understand is, what would make one to think that 

8 hours ago, D Martin said:

the genetic variation/mutation would not allow for normal development. 

 

[CharonY]

Yep. Plenty of diseases with a genetic basis manifest later in life and might have little or no effect during development. There are many potential reasons, some include being inactive until later in life, requirement of some interactions (either internally or with the environment) to manifest the disorder, functions that can inhibit the disorder during development but not later in life, etc.

One might think that organisms are extremely purpose-built and small changes can cause disorder or malfunction. In reality what is considered "normal" is more of a weird mess where a lot of variation exists that under certain conditions works perfectly fine but may break down under others. It is much less deterministic what works and what doesn't as one might imagine.

[TheVat]

Coeliac is the poster child of elusive diseases that may not manifest until well into adulthood (though there is a pediatric form that can be diagnosed quite early nowadays).  Because it involves a complex interaction between the immune system and a protein found in wheat or rye or barley and the lining of the small intestine, it can range from no symptoms (until you get bowel cancer later in life, or finally show malabsorption of nutrients later in life due to very gradual degradation of intestinal villi) to complete havoc - severe IBS-like misery, wasting, anemia, and such.  In children, there can be serious developmental consequences including short stature, neurological effects like cerebellar ataxia, delayed puberty, etc.  There is also some indication that exposures to certain agricultural chemicals in wheat growing may trigger some autoimmune responses like coeliac.

So, to respond to OP, this illustrates that gene interactions with our environment are pretty complex.  It may take decades before certain genes, like the HLA-DQA1 and HLA-DQB1 gene variants implicated in coeliac, are activated in a way to cause trouble.  (btw, you can get a test for the two principal gene variants by mail for around 100-150 USD, which may be well worth doing)